Implement VLSEffect

[vanruesc]

Description

Implement a VLSEffect (volumetric light scattering, god rays, crepuscular rays).

References:

• https://github.com/mrdoob/three.js/blob/0bf3908b73b2cf73d7361cce17cfc8b816cb2a00/examples/jsm/shaders/GodRaysShader.js
• https://developer.nvidia.com/gpugems/gpugems3/part-ii-light-and-shadows/chapter-13-volumetric-light-scattering-post-process
• https://doc.babylonjs.com/features/featuresDeepDive/lights/volumetricLightScattering

Tasks

• [ ] Create RadialBlurPass.
• [ ] Create VLSEffect.
• [ ] Create vls demo.
• [ ] Add unit test.

Implementation Details

• Use a multi-pass radial blur.
• The implementation from v6 can be used for reference.
• The effect should use a default light source if none is provided.
  • The mesh should be a billboard (white radial gradient).
  • Provide a method for creating colored billboards.