Fix unified gsplat work buffer extra streams for octrees and color-only updates

[mvaligursky]

Fixes two related issues with unified gsplat work buffer extra streams:

1. Octree parent placement delegation

Octree file placements now properly inherit shader configuration from the component's placement, enabling them to write to work buffer extra streams:

• workBufferModifier - delegated via getter (allows writing to extra streams)
• parameters - inherited in constructor (e.g., component ID uniforms)
• streams (instance textures) - delegated via getter

This allows octree splats to use per-component parameters and modifiers when writing to work buffer extra streams

2. Color-only shader compilation fix

When camera movement triggers a color-only update (spherical harmonics recalculation), the work buffer shader now generates no-op stub functions for extra stream writes. This prevents shader compilation errors when the user's workBufferModifier calls functions like writeSplatComponentId() that don't have render targets in color-only mode.

[Copilot]

Pull request overview

This PR fixes two issues with unified gsplat work buffer extra streams:

Changes:

• Added parent placement delegation for octree file placements to inherit shader configuration (workBufferModifier and instance streams)
• Implemented no-op stub generation for extra stream write functions in color-only mode to prevent shader compilation errors

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/scene/shader-lib/wgsl/chunks/gsplat/vert/gsplatStreamOutput.js	Added {defineGuard} placeholder to enable conditional compilation of stream output
src/scene/shader-lib/glsl/chunks/gsplat/vert/gsplatStreamOutput.js	Added {defineGuard} placeholder to enable conditional compilation of stream output
src/scene/gsplat/gsplat-format.js	Added getOutputStubs method to generate no-op stub functions for streams
src/scene/gsplat/gsplat-resource-base.js	Modified to generate stub functions for extra streams in color-only mode
src/scene/gsplat-unified/gsplat-placement.js	Added parent placement support with delegation for workBufferModifier and streams getters
src/scene/gsplat-unified/gsplat-octree-instance.js	Updated file placement creation to pass parent placement reference
src/scene/gsplat-unified/gsplat-info.js	Changed instanceStreams from property to getInstanceStreams getter function for live retrieval
src/scene/gsplat-unified/gsplat-work-buffer-render-pass.js	Updated to use getInstanceStreams function for live stream access

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