added check for root (#3545) ...

* added check for root

* Added note to future us..

remove `@private` of `updateTransform`. (#3553) ...

In some complex use case ， users would call updateTransform manually.

And , if  `renderWebGL` is public , `updateTransform` should be public too.

Merge pull request #3548 from pixijs/improved-docs ...

Improved documentation

Add readonly to view

Creates new PIXI.Application class

Improved documentation ...

Class member documentation improved via removal of unnecessary @memberofs - more members now show up in the docs.
Class getter/setter documentation improved using technique above, and also by removing documentation for setters. Documentation was only showing the setter docs, whereas the getter is where the important information is. JSDoc maintainers themselves recommend just doccing the getter jsdoc3/jsdoc#973
Documented a few classes to let devs know that they are automatically created by PIXI and added to renderer.plugins, like interactionManager and prepare

Fix a bug about compute Text canvas size. (#3519) ...

* Fix a bug about compute Text canvas size.

There are 2 bugs :
* should NOT use `this.context.lineWidth`  before `this.context.lineWidth = style.strokeThickness`.
* the default value of `style.strokeThickness` is `0` , but  `this.context.lineWidth` couldn't be 0 ( in HTML5 , context.lineWidth = 0 will be ignore). So we should NOT use `this.context.lineWidth` for computing size.

* Update Text.js

Don't need  ` + style.strokeThickness` again.

* Update Text.js

* Update Text.js

refactor :   change the  codes of computing `canvas.width` & `canvas.height`  in the same code style.

* use  `style` instead of `this._style`

use high precision instead of medium, to fix bug in PowerVR G6200 (#3353)

Improved documentation of the destroy method (#3529) ...

* Improved documentation of the destroy method
Documents the .texture and .baseTexture parameters for the settings object that may not be used for the class itself, but could get propogated down to children
Fixes https://github.com/pixijs/pixi.js/issues/3407

* Improved clarity of docs

fix eslint max-len (#3544)

fix for invalid texture (#3474) ...

* fix for invalid texture

* new approach to fix

undo last fix
add fix into texture manager

* added break (thanks @finscn)

* fixed little typo

* implemented @englercj tweaks

* on reflection, this should never be null!

Fixes CanvasRenderer getting stuck at the last renderTexture resolution (#3530) ...

Fixes https://github.com/pixijs/pixi.js/issues/3135 and https://github.com/pixijs/pixi.js/issues/3359
Still allows dynamic changing of resolution which introduced this bug in the first place: https://github.com/pixijs/pixi.js/pull/2870

fixed some filter issues with textures not clearing correctly (#3525) ...

* fixed some filter issues with textures not clearing correctly

* removed commented code

* added a clear to the correct place (when filters are stacked)

Add clear method to CanvasRenderer (#3423) ...

* Add clear method to CanvasRenderer

WebGLRenderer has the ```clear``` method , I think CanvasRenderer should has it too.

Sometimes, maybe we want to clear the canvas manually via ``` game.renderer.clear()``` .

If  CanvasRenderer doesn't  has this method ,  there will be an error.

* Update CanvasRenderer.js

Merge branch 'master' into dev

Remove the needless checking for `orig` (#3479) ...

In Texture , the `orig` is always exist after new Texture object . 

 If `orig` is not accessible , there will be a bug in the App.
The developers must fix it , not ignore and return `0`.

Merge pull request #3502 from finscn/patch-22 ...

The canvas could be passed into Text

Merge pull request #3513 from pixijs/multi-font-support ...

Adds support for multiple font families

Fixed doc style for typed array

Use `*._texture.*` instead of`*.texture.*` (#3517) ...

It could offer a bit little performance gains.

and in current version  https://github.com/pixijs/pixi.js/blob/master/src/core/text/Text.js#L637  , there is already used `_texture` .
So I think this patch is OK.

Less codes , Less assignment operations. ...

Remove the needless assignment operation.

Adds support for multiple font families

Use `*._texture.*`to instead of`*.texture.*` (#3508) ...

it could offer a bit little performance gains.

limit alpha number to 255. at rendering. (#3511) ...

* limit alpha number to 255. at rendering.

Avoid alpha flickering when it passes 1.0

* added space after comma

Use `this._texture.X`to instead of`this.texture.X` (#3480) ...

In get/set width/height method , we don't need use  `this.texture` .

We could use `this._texture.X`to instead of`this.texture.X` , and it could offer a bit little performance gains.

Remove a needless space in annotation (#3497)

Protect the initial state of `this._activeRenderTarget` in _indexContext (#3443) ...

* Update WebGLRenderer.js

```
    this._activeRenderTarget = null;
    this._activeTextureLocation = 999;
    this._activeTexture = null;
````
must before `this._indexContext()` , otherwise  the _activeRenderTarget & _activeTexture init in  _indexContext will back to `null`.

* Add a blank line for cosmetic.

The canvas could be passed into Text ...

This feature could help users to  share canvas among two or more  Text  objects （ like Graphics._SPRITE_TEXTURE in Graphics).

In the game ,normally  we need rebuild stage/scene when stage/scene changed ,  If Text supports passing canvas , we could avoid create and destroy HTMLCanvasElement often when game running. 

e.g. create a canvas-pool  cache some canvas for Text.

Update annotation: `SpriteBatch to SpriteRenderer` (#3496) ...

* Update annotation: `SpriteBatch to SpriteRenderer`

PIXP 4.x  no SpriteBatch

* more

Update `clear` for a more thorough cleaning (#3445) ...

* Update `clear` for a more thorough cleaning

Clear something that should be cleared.
`
        this.currentPath = null;
        this._spriteRect = null;
`
if no this patch , some test cases will be failed.


##Test Case
There is some common codes of test cases :
```js
var renderer = new PIXI.WebGLRenderer(900, 600, {
    clearBeforeRender: false,
    backgroundColor: 0xFFFFFF
});
document.body.appendChild(renderer.view);

// the root container.
var stage = new PIXI.Container();
```

### TEST 1
If no  ```currentPath = null ``` ,  the second arc can't be rendered, and there is no error on browser's console.

```js

    var graphics = new PIXI.Graphics();
    stage.addChild(graphics);

    // clear manually
    renderer.clear();

    graphics.clear();
    graphics.beginFill(0xFF0000);
    graphics.arc(150, 160, 100, 0, Math.PI * 2, false);
    graphics.endFill();

   // draw  the first shape
    renderer.render(stage);

    graphics.clear();
    graphics.beginFill(0x0000FF);
    graphics.arc(150+50, 160+50, 100, 0, Math.PI * 2, false);
    graphics.endFill();

   // draw  the second shape
    renderer.render(stage);
```

### TEST 2
If no  ```_spriteRect = null ``` ,  the first rect can't be rendered, and there is no error on browser's console.

```
    var graphics = new PIXI.Graphics();
    stage.addChild(graphics);

    // clear manually
    renderer.clear();

    graphics.clear();
    graphics.beginFill(0xFF0000);
    graphics.drawRect(150, 160, 100, 60);
    graphics.endFill();

    // draw the first shape
    graphics.updateTransform();
    graphics.renderWebGL(renderer);

    graphics.clear();
    graphics.beginFill(0x0000FF);
    graphics.drawRect(150+50, 160+50, 100, 60);
    graphics.endFill();

    // draw the second shape
    graphics.updateTransform();
    graphics.renderWebGL(renderer);

    // flush manually
    renderer.flush();
```

### Summary

If don't call  `graphics.clear();` before draw the second shape , there will be another error.

If add 
```
        this.currentPath = null;
        this._spriteRect = null;
```
in `clear()` everything will be OK, and I test many other examples, this patch doesn't break anything else, it's safe.


I know when we want to draw multi shapes , we could use multi graphics objects.
But we shouldn't prevent users to use one shared graphics to draw multi shapes.

* Remove trailing spaces.