Further gsplat shader refinements

[slimbuck]

This PR is a followup to #7169 and further refines the chunk layout for gsplat material:

• Gaussians now have 3 data structures:
  • SplatSource - stores the splat's data uv, id and render order
  • SplatCenter - stores the splat's center in camera and clip space
  • SplatCorner - stores the splat's corner offset (from center) in clip space and the corner uv
• The functions initSource, initCenter and initCorner populate the respective structures and are placed in their own chunks for easy customisation
• Each chunk now declares and uses its own global variables (uniforms and attributes)

Also:

• Reflection vector for SH evaluation is done in the main function so it can easily be customised
• Slightly faster early-out when gaussian is behind the camera
• Disable clipping of splats by the near and far clipping planes (clamp depth instead)
• Replace tex_params uniform with call to glsl textureSize

[mvaligursky]

With clustered lighting, I had precision issue when a mat4 was stored in a struct, so I had to move it out. Not sure it would have the same impact here, likely not as shadow flickering is a specific issue.