Cleanup BatchRenderer internal names and documentation (#5852) ...

I have edited the BatchRenderer class. The following major changes were
made:

1. Changed old `vertexCount` to `_flushId`. This is because the name
itself was misleading. It signified the number of flushes in the
current frame.

2. Renamed `vaos` and `vaosMax` to `_packedGeometries` and
`_packedGeometryPoolSize`. They _were not_ VAOs in the sense
that they binded all the buffers automatically to the WebGL
context. Instead, they were just reusable `BatchGeometry` objects.

3. Renamed `groups` to `_drawCalls`. That is what they are, and it
makes more sense, especially after documenting it.

4. Renamed all private variables by adding a _ prefix.

5. Added documentation to all private/public properties and methods.

6. Fixed destroy(). It was nullifying non-existent variables. They
were probably from when this class was being built/tested. I've fixed
it so that it nullifies the correct resources.

7. contextChange signal is now hooked in `BatchRenderer` rather than
the `BatchPluginFactory#create`. This makes sense since the former
is the one implementing the `contextChange` signal.

8. SIDE EFFECT: Graphics used the `shader` property of `BatchRenderer`,
and since that was deemed a private properties, it broke. It fixed that
by making Graphics use `_shader`.

9. I reordered the way in which properties are initialized in the
constructor. The most important/public ones are on the top. They are
ordered in a way that makes a new-comer sense.

10. Reordered methods. The first methods are related to rendering and
are lifecycle-type methods: contextChange, render, flush, and destroy. Then
the start and stop methods. Then the private methods getAttributeBuffer,
getIndexBuffer, and packInterleavedGeometry.

11. Renamed packGeometry to packInterleavedGeometry; this is because
the method doesn't take any geometry to put into. Rather it takes
the interleaved buffers.

12. Documented literally anything else.

Fix for the empty Graphics batchability issue (#5821)

Fixes Graphics clear fill and line style settings (#5797)

Renames StateSystem.setState to StateSystem.set (#5791)

Fixes beginTextureFill texture region height (#5648)

Assure that TICK is different for multi-textured renderers (#5546)

Fixes for roundedRect and circle stroke alignment (#5567)

Graphics documentation and typing fixes (#5544)

Remove unused field, fix typings (#5527)

Fixes arc duplicating points (#5526)

Adds partial support for texture regions with beginTextureFill (#5519)

Fixes Graphics holes on canvas and closePath (#5516)

Fixes Graphics closePath (#5515)

Fixes Graphics clear (#5514)

Fixes detecting close stroke shapes (#5453)

Fix Adaptive curve formulas (#5442)

Fixes non-batched Graphics (#5401)

Proper disposal for Mesh and Graphics Geometry objects (#5349)

Documentation updates (#5366)

Fix types in documentation (#5345)

Fixes beginTextureFill and lineTextureStyle usage of Matrix (#5295)

Excludes GL lines from batching (#5294)

Fixes native lines in lineStyle and adds BatchDrawCall class (#5293)

Re-enabled Graphics batching (#5264)

Fixes moveTo and lineStyle (#5221)

Fixed tint / alpha for non-batched Graphics objects (#5217)

Graphics does not depend on Mesh (#5206)

Replaces toJSON with clone in Graphics LineStyle (#5207)

Graphics memory leak (#5203)

Tweaked graphics to share one default shader (#5205)