PixelationPassNode: Simplify Code.

[cmhhelgeson]

Description

Simplifies the PixelationPassNode code by properly using getTextureNode and textureSize. Additionally adds some comments to specify how the effect might differ from RetroPassNode.

[Mugen87]

I've extended the refactoring with a few additions and corrections:

• The pixelSize is now a number only. It does not make sense to allow a TSL object since the value is never used in the shader.
• Use toConst()/toVar() where appropriate.
• Minor JSDoc and typo corrections.

The pixelSize change is a breaking one, I'll note that in the migration guide.