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

Use `child._texture` instead of `child.texture`. (#3532) ...

For better performance ,  a bit little  , but worth.

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`.

A few doc updates (#3527)

Added MockPointer (#3475) ...

* refactor to MockPointer

* add onTap tests

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

Merge pull request #3512 from englercj/dev ...

Update to Resource loader v2; fixes #2876

Add simple tests for loader

correct blob parser import

Update to Resource loader v2; fixes #2876

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.

Merge pull request #3515 from finscn/patch-25 ...

Less codes , Less assignment operations.

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

Make ignore more strict

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.

docs fix for TextureTransform (#3498)

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

* Update annotation: `SpriteBatch to SpriteRenderer`

PIXP 4.x  no SpriteBatch

* more

Code Formatting (#3484) ...

Format some codes.

Haven't changed any logic.

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.

added InteractionManager tests (#3472)

Test case & fix for Graphics._calculateBounds calling updateLocalBounds every time (#3444) ...

* Test case for #3414

* Fixes #3414

* fixed expectation