Refactor GSplatRenderer into base class with GSplatQuadRenderer derived class

[mvaligursky]

Refactor GSplatRenderer into a thin base class that holds common state (device, node, cameraNode, layer, workBuffer, renderMode) and a derived GSplatQuadRenderer that contains all instanced-quad rendering logic (material, mesh instance, blend modes, indirect draw). This prepares the architecture for future compute-shader-based splat renderers that won't use MeshInstance at all.

The base class defines the public renderer API as stub methods, allowing GSplatManager to work through the base type. GSplatQuadRenderer overrides all methods with the existing instanced-quad implementation.

Also fixes a pre-existing bug where switching from GPU to CPU sorting could render a few frames with stale order data before the CPU worker delivers fresh sort results.

Changes:

• Split GSplatRenderer into a base class with common state and stub API methods
• Create GSplatQuadRenderer extending the base with all material/mesh/instancing logic
• Update GSplatManager to type its renderer as GSplatRenderer (base type) while constructing GSplatQuadRenderer
• Move visibility hiding into GSplatQuadRenderer.disableIndirectDraw() instead of external calls
• Guard work buffer updates and frame updates against stale sorted state (sortedBefore === false) to prevent incorrect frames during GPU→CPU sort transitions

[Copilot]

Pull request overview

Refactors the unified GSplat rendering implementation by splitting the previous monolithic GSplatRenderer into a shared-state base class and a concrete instanced-quad renderer, while also tightening update logic to avoid rendering with stale sort/order data during GPU→CPU sorting transitions.

Changes:

• Split GSplatRenderer into a thin base/interface holding shared renderer state and API surface.
• Added GSplatQuadRenderer implementing the existing instanced-quad + MeshInstance rendering path (materials, defines, indirect draw).
• Updated GSplatManager to construct GSplatQuadRenderer and to gate work-buffer/frame updates when the current sorted state hasn’t produced valid sort results yet (sortedBefore === false).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/scene/gsplat-unified/gsplat-renderer.js	Converts renderer into a base class with shared state and stubbed API methods.
src/scene/gsplat-unified/gsplat-quad-renderer.js	New derived renderer containing all instanced-quad rendering logic previously in GSplatRenderer.
src/scene/gsplat-unified/gsplat-manager.js	Instantiates GSplatQuadRenderer, moves indirect-draw visibility handling into the renderer, and guards updates against stale sorted state.

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