Add gsplatModifyVS shader customization chunk

Adds a new shader customization API for Gaussian splats that works directly with rotation and scale instead of covariance, providing better performance and a simpler API. This is non-breaking change, and the existing chunk override functions (with a deprecated warning)

New Features

• New gsplatModifyVS shader chunk with three hook functions:

  • modifySplatCenter(inout vec3 center) - modify splat position
  • modifySplatRotationScale(vec3 originalCenter, vec3 modifiedCenter, inout vec4 rotation, inout vec3 scale) - modify rotation quaternion and scale
  • modifySplatColor(vec3 center, inout vec4 color) - modify splat color

• New helper functions in gsplatHelpers:

  • gsplatMakeSpherical(inout vec3 scale, float size) - make splat spherical by setting uniform scale
  • gsplatGetSizeFromScale(vec3 scale) - get RMS size from scale vector

• Shader chunk validation system via ShaderChunks.registerValidation():

  • Deprecation warnings when old chunks are overridden
  • Support for custom validation callbacks
  • Separate GLSL/WGSL default code comparison
  • Works correctly with app recreation scenarios

The old gsplatCustomizeVS chunk remains functional but is deprecated. A warning is logged when it's overridden. To migrate:

Updated Examples / Scripts

• multi-splat.example.mjs - uses new gsplatModifyVS
• All gsplat shader effect scripts converted to scale-based API:
  • reveal-radial.mjs
  • reveal-rain.mjs
  • reveal-grid-eruption.mjs
  • shader-effect-box.mjs
  • shader-effect-crop.mjs