Replace interval texture binary search with instanced quad rendering for GSplat work buffer copy

[mvaligursky]

Replaces the two-pass interval texture expansion (GSplatIntervalTexture class + binary search
fragment shader) with a single instanced draw call when copying GSplat data to the work buffer.

Functionality:

• Each LOD interval is split into row-aligned rectangles on the CPU and stored in a small
  RGBA32U data texture. One instanced quad is drawn per rectangle, with the fragment shader
  computing the original source index arithmetically from flat varyings — eliminating the
  binary search texture lookup entirely.
• QuadRender unified to always use indexed PRIMITIVE_TRIANGLES via the shared quadIndexBuffer,
  with a new numInstances parameter for instanced draws.

[Copilot]

Pull request overview

This PR replaces the two-pass GPU-based interval texture expansion (using binary search) with a more efficient single-pass instanced rendering approach for copying GSplat data to the work buffer. The new implementation computes source indices arithmetically in the fragment shader, eliminating the need for the intermediate interval texture and binary search lookup.

Changes:

• Removed GSplatIntervalTexture class and associated binary search fragment shaders
• Added instanced quad vertex shaders that split LOD intervals into row-aligned rectangles on the CPU
• Modified fragment shaders to compute original splat indices arithmetically from flat varyings
• Unified QuadRender to always use indexed PRIMITIVE_TRIANGLES with a new shared quadIndexBuffer
• Added numInstances parameter to QuadRender for instanced drawing support

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/scene/shader-lib/wgsl/chunks/gsplat/vert/gsplatCopyInstancedQuad.js	New WGSL vertex shader for instanced quad rendering with per-instance rectangle parameters
src/scene/shader-lib/glsl/chunks/gsplat/vert/gsplatCopyInstancedQuad.js	New GLSL vertex shader for instanced quad rendering with per-instance rectangle parameters
src/scene/shader-lib/wgsl/chunks/gsplat/frag/gsplatIntervalTexture.js	Removed - binary search interval expansion no longer needed
src/scene/shader-lib/glsl/chunks/gsplat/frag/gsplatIntervalTexture.js	Removed - binary search interval expansion no longer needed
src/scene/shader-lib/wgsl/chunks/gsplat/frag/gsplatCopyToWorkbuffer.js	Updated to compute source index arithmetically from flat varyings instead of texture lookup
src/scene/shader-lib/glsl/chunks/gsplat/frag/gsplatCopyToWorkbuffer.js	Updated to compute source index arithmetically from flat varyings instead of texture lookup
src/scene/gsplat-unified/gsplat-work-buffer.js	Updated to select instanced vertex shader for LOD path, removed unused device field
src/scene/gsplat-unified/gsplat-work-buffer-render-pass.js	Updated to use instanced rendering with sub-draw data texture instead of interval texture
src/scene/gsplat-unified/gsplat-interval-texture.js	Removed - entire class replaced by simpler sub-draw data texture approach
src/scene/gsplat-unified/gsplat-info.js	Added updateSubDraws method to split intervals into row-aligned rectangles and pack into texture
src/scene/graphics/quad-render.js	Unified to use indexed triangles, added numInstances parameter for instanced rendering
src/platform/graphics/graphics-device.js	Added quadIndexBuffer for indexed quad rendering

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