renamed BatchBuffer to ViewableBuffer in geometry.

This commit makes two major changes:

1. Adds attribute definitions to AbstractBatchRenderer.

2. Renames the BatchPluginFactory to BatchRendererFactory, since it
doesn't actually "plug in" the renderer. That is done elsewhere.

rawBinaryData, since that is what an ArrayBuffer is.

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.