Large TextureTintPipeline refactor

* The Texture Tint vertex and fragment shaders have been updated to support the `inTexId` float attribute and dynamic generation.
* The Texture Tint Pipeline has a new attribute, `inTexId` which is a `gl.FLOAT`.
* `TextureTintPipeline.bind` is a new method that sets the `uMainSampler` uniform.
* The `TextureTintPipeline.requireTextureBatch` method has been removed, as it's no longer required.
* The `TextureTintPipeline.pushBatch` method has been removed, as it's no longer required.
* The `TextureTintPipeline.maxQuads` property has been removed, as it's no longer required.
* The `TextureTintPipeline.batches` property has been removed, as it's no longer required.
* `TextureTintPipeline.flush` has been rewritten to support multi-textures.
* `TextureTintPipeline.flush` no longer creates a sub-array if the batch is full, but instead uses `bufferData` for speed.
* `TextureTintPipeline.currentUnit` is a new property that holds the most recently assigned texture unit. Treat as read-only.

Author: photonstorm