Compact GSplat work buffer format

[mvaligursky]

Adds a compact 20 bytes/splat work buffer format (GSPLATDATA_COMPACT) as the new default,
reducing memory and bandwidth by 37.5% compared to the existing 32 bytes/splat format.

• Compact format layout: R32U color (11+11+10 bit RGB, [0,4] range), RGBA32U transform
  (f32 position + half-angle quaternion), R32U scale/alpha (3x8-bit log-encoded scale + 8-bit alpha)
• Half-angle quaternion projection for rotation: branchless decode, no index bits overhead
• Log-encoded scale covering e^-12 to e^9 range
• Switchable at runtime via scene.gsplat.dataFormat property
• Unified instanced draw path for all splats (octree and non-octree), removing the
  zero-fill code path and redundant uniforms
• Format-specific writeSplat() shader interface decoupling the copy shader from
  individual format encodings
• Compact toggle added to viewer and lod-streaming examples

[Copilot]

Pull request overview

This PR introduces a compact work buffer format for GSplat rendering that reduces memory and bandwidth usage by 37.5% (from 32 to 20 bytes per splat) compared to the existing format. The compact format uses advanced encoding techniques including 11+11+10 bit RGB color (supporting [0,4] range), half-angle quaternion projection for rotation (3 components instead of 4), and log-encoded scale values covering e^-12 to e^9 range. The format is runtime-switchable via the scene.gsplat.dataFormat property, with GSPLATDATA_COMPACT as the new default and GSPLATDATA_LARGE available for full precision when needed.

The PR also unifies the rendering code path by using instanced draws for all splats (both octree and non-octree), eliminating the zero-fill code path that was previously needed for non-LOD splats. This simplification removes redundant uniforms (uActiveSplats, uViewportWidth) and ensures consistent behavior across all rendering scenarios.

Changes:

• Added compact 20 bytes/splat format with specialized encoding (half-angle quaternion, log-encoded scale, packed RGB)
• Created format-specific read/write shader interfaces for both GLSL and WGSL
• Unified instanced draw path for all splats, removing conditional rendering logic
• Added runtime format switching capability with automatic work buffer recreation
• Updated viewer and lod-streaming examples with compact format toggle (defaulting to enabled)

Reviewed changes

Copilot reviewed 22 out of 22 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/scene/constants.js	Added GSPLATDATA_COMPACT and GSPLATDATA_LARGE constants for format selection
src/scene/gsplat-unified/gsplat-params.js	Implemented format creation and runtime switching logic, defaulting to compact format
src/scene/gsplat-unified/gsplat-manager.js	Added handleFormatChange() to detect and handle format changes, ensuring work buffer recreation
src/scene/gsplat-unified/gsplat-work-buffer-render-pass.js	Removed conditional intervals logic, unified to always use instanced draws
src/scene/gsplat-unified/gsplat-renderer.js	Extracted setOrderData() method for reuse after work buffer changes
src/scene/gsplat-unified/gsplat-info.js	Synthesizes full-range interval when none exist to ensure unified instanced path
src/scene/gsplat/gsplat-format.js	Added setWriteCode/getWriteCode methods for format-specific write encoding
src/scene/gsplat/gsplat-resource-base.js	Integrated format-specific write code injection, removed useIntervals parameter
src/scene/shader-lib/wgsl/chunks/gsplat/vert/formats/containerCompactRead.js	WGSL read functions for compact format with half-angle quaternion and log-scale decoding
src/scene/shader-lib/wgsl/chunks/gsplat/frag/formats/containerCompactWrite.js	WGSL write functions for compact format with specialized encoding
src/scene/shader-lib/wgsl/chunks/gsplat/frag/formats/containerPackedWrite.js	WGSL write functions for large format, extracted from copy shader
src/scene/shader-lib/wgsl/chunks/gsplat/frag/gsplatCopyToWorkbuffer.js	Unified shader logic, removed zero-fill path, uses format-specific writeSplat()
src/scene/shader-lib/glsl/chunks/gsplat/vert/formats/containerCompactRead.js	GLSL read functions for compact format (parallel to WGSL version)
src/scene/shader-lib/glsl/chunks/gsplat/frag/formats/containerCompactWrite.js	GLSL write functions for compact format (parallel to WGSL version)
src/scene/shader-lib/glsl/chunks/gsplat/frag/formats/containerPackedWrite.js	GLSL write functions for large format, extracted from copy shader
src/scene/shader-lib/glsl/chunks/gsplat/frag/gsplatCopyToWorkbuffer.js	Unified shader logic (parallel to WGSL version)
src/scene/shader-lib/wgsl/collections/gsplat-chunks-wgsl.js	Removed containerPackedRead from collection (now imported directly where needed)
src/scene/shader-lib/glsl/collections/gsplat-chunks-glsl.js	Removed containerPackedRead from collection (now imported directly where needed)
examples/src/examples/gaussian-splatting/viewer.example.mjs	Added compact toggle control, defaults to enabled
examples/src/examples/gaussian-splatting/viewer.controls.mjs	Added UI control for compact format toggle
examples/src/examples/gaussian-splatting/lod-streaming.example.mjs	Added compact toggle control, defaults to enabled
examples/src/examples/gaussian-splatting/lod-streaming.controls.mjs	Added UI control for compact format toggle

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