Add support for velocity rendering

[vanruesc]

Description

Implement velocity rendering by extending ShaderLib with MRT shader code based on three's VelocityShader.

References:

• https://github.com/mrdoob/three.js/blob/74d7dc8a5fb92e40758ec6b8fd08502c5ca019b3/examples/jsm/shaders/VelocityShader.js
• https://github.com/mrdoob/three.js/blob/74d7dc8a5fb92e40758ec6b8fd08502c5ca019b3/examples/webgl_materials_channels.html
• https://john-chapman-graphics.blogspot.com/2013/01/per-object-motion-blur.html
• https://github.com/mrdoob/three.js/pull/29058/files

Tasks

• Create and register the required pp_gbuffer_velocity_* shader chunks.
• Update ShaderChunkExtensions to assign the shader chunks to three's materials.
• Add GBuffer.VELOCITY and GData.VELOCITY.
• Update GeometryPass to define a velocity texture config.
• Update GBufferConfig, ClearFlags and ClearValues.
• Update gbuffer demo to render velocity.