Fix splat overlay rendering performance

[slimbuck]

Summary

• Fix the splat overlay creating and leaking a new WebGL VAO on every frame by adding a dummy 1-vertex instancing VB with SEMANTIC_POSITION, which makes the engine cache the VAO on the vertex buffer
• Remove per-frame material.update() call from onPreRender() — only setParameter() is needed for uniform changes; update() was clearing shader variant caches every frame due to ShaderMaterial._dirtyShader being permanently true
• Remove gl_FragDepth and vZW varying from the overlay shader — depthWrite is false so the written depth was unused, and gl_FragDepth disables Early-Z optimization

Details

The overlay mesh uses attributeless PRIMITIVE_POINTS rendering via gl_VertexID (0 vertex buffers). The engine's setBuffers only caches VAOs for the single-VB path (vertexBuffers.length === 1); the 0-VB case falls through to createVertexArray which has useCache = vertexBuffers.length > 1, so a brand new WebGL VAO was created and leaked every frame. On macOS with ANGLE (WebGL-to-Metal), this caused significant overhead from repeated Metal pipeline state lookups.

The workaround adds a dummy 1-vertex VB with format.instancing = true (sets vertexAttribDivisor(loc, 1)) so only 1 vertex is needed regardless of draw count. This also resolves the "No vertex attribute is mapped to location 0" Safari compatibility warning.

A proper engine fix (caching an empty VAO for the 0-VB case in setBuffers) will follow separately.

[Copilot]

Pull request overview

This PR targets a performance leak in the splat overlay rendering path by ensuring the engine can reuse/caches the WebGL VAO for the overlay draw and by removing unnecessary per-frame shader/material work.

Changes:

• Add a dummy 1-vertex VertexBuffer to the overlay Mesh so PlayCanvas hits the VAO caching path (avoiding per-frame VAO creation/leak).
• Remove per-frame material.update() from SplatOverlay.onPreRender() and rely on setParameter() for uniform updates.
• Simplify the overlay shader by removing the vZW varying and gl_FragDepth write.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/splat-overlay.ts	Adds a dummy vertex buffer to enable VAO caching and removes per-frame material.update() calls.
src/shaders/splat-overlay-shader.ts	Removes custom fragment depth output and an unused varying to reduce shader overhead.

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