Implement PixelationEffect

[vanruesc]

Description

Implement a PixelationEffect.

References:

• https://github.com/mrdoob/three.js/blob/0bf3908b73b2cf73d7361cce17cfc8b816cb2a00/examples/jsm/postprocessing/RenderPixelatedPass.js#L13

Tasks

• Create PixelationEffect.
• Create pixelation demo.
• Add unit test.

Implementation Details

• Use G-Buffer textures (depth, normals).
• Check if EdgeDetectionMaterial from SMAA can be reused.
• Check if normal/depth outline code overlaps with OutlineEffect.