Remove per-resource instance vertex buffer from GSplat rendering

[mvaligursky]

Replaces the per-resource instance vertex buffer (vertex_id_attrib) in GSplat rendering with attributeless instancing, deriving the splat base index from gl_InstanceID / pcInstanceIndex GPU builtins instead.

Changes:

• Remove createInstanceIndices and all per-resource instance VB allocation/reallocation from GSplatResourceBase
• Remove setMaxNumSplats dynamic reallocation from GSplatRenderer
• Update GLSL and WGSL gsplatSource shaders to compute source.order from the instance index builtin multiplied by a GSPLAT_INSTANCE_SIZE define
• Apply the same changes to both the unified (GSplatRenderer) and legacy (GSplatInstance) rendering paths

API Changes:

• MeshInstance.setInstancing now accepts true as the vertexBuffer parameter to enable attributeless instancing (count-only, no per-instance vertex buffer). The caller must set instancingCount manually.

Performance:

• Eliminates per-resource GPU memory allocation for instance index vertex buffers
• Removes dynamic reallocation when total splat count grows

[Copilot]

Pull request overview

This PR refactors GSplat rendering to remove the per-resource instance-index vertex buffer and instead derive the base splat index from the GPU instance index builtins (gl_InstanceID / pcInstanceIndex), reducing GPU allocations and reallocations as splat counts change.

Changes:

• Switched GSplat shaders (GLSL/WGSL) to compute source.order from the instance index builtin and a {GSPLAT_INSTANCE_SIZE} replacement define.
• Removed per-resource instance index vertex buffer creation/ownership from GSplatResourceBase and removed dynamic resizing logic from the unified renderer/manager path.
• Extended MeshInstance.setInstancing to accept true for attributeless instancing (instance-count only).

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/gsplatSource.js	Uses pcInstanceIndex + {GSPLAT_INSTANCE_SIZE} to derive splat order (no vertex_id_attrib).
src/scene/shader-lib/glsl/chunks/gsplat/vert/gsplatSource.js	Uses gl_InstanceID + {GSPLAT_INSTANCE_SIZE} to derive splat order (no vertex_id_attrib).
src/scene/mesh-instance.js	Adds attributeless instancing mode via setInstancing(true, ...).
src/scene/gsplat/gsplat-resource-base.js	Removes per-resource instance index VB allocation and lifetime management.
src/scene/gsplat/gsplat-material.js	Drops vertex_id_attrib attribute binding; injects {GSPLAT_INSTANCE_SIZE} define.
src/scene/gsplat/gsplat-instance.js	Switches legacy path to attributeless instancing; injects {GSPLAT_INSTANCE_SIZE} define during configuration.
src/scene/gsplat-unified/gsplat-renderer.js	Switches unified path to attributeless instancing; removes setMaxNumSplats; injects/protects {GSPLAT_INSTANCE_SIZE} define.
src/scene/gsplat-unified/gsplat-manager.js	Removes calls to the deleted renderer.setMaxNumSplats.

Comments suppressed due to low confidence (1)

src/scene/gsplat-unified/gsplat-renderer.js:426

• In indirect-draw mode (when draw commands are provided), instancingCount is GPU-driven and numInstances is ignored by WebgpuGraphicsDevice.draw(), but the forward/shadow renderers still skip any draw call where instancingData.count <= 0. With attributeless instancing, setInstancing(true, ...) initializes InstancingData with count = 0 and createMeshInstance() leaves instancingCount = 0, so indirect GSplat rendering will be skipped entirely. Consider setting meshInstance.instancingCount to a non-zero sentinel (for example 1) when indirect draw is enabled (e.g. in setIndirectDraw / updateIndirect) or initializing attributeless instancing with a default count of 1 for indirect-driven paths.

        const mesh = GSplatResourceBase.createMesh(this.device);
        const meshInstance = new MeshInstance(mesh, this._material);
        meshInstance.node = this.node;
        meshInstance.setInstancing(true, true);

        // only start rendering the splat after we've received the splat order data
        meshInstance.instancingCount = 0;

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