WebGLRenderer: Prerender transmission pass(es)

[mrxz]

Related issue: #22594 (split off from #28078)

Description
Rendering the transmission pass happened at the start of renderScene. In case of WebXR the scene is rendered twice sequentially, once for the left eye and once for the right eye. This resulted in the corresponding transmission passes to be interleaved with the main rendering (see ovrgpuprofiler output in #28078 and #22594). These additional switches cause expensive flushes to, and loads from main memory.

This PR moves the rendering of the transmission pass before renderScene. For WebXR this allows the transmission buffer to be rendered for the left and right eye in succession, after which the left and right eye can be rendered as normal without needing to switch render target in-between.

This contribution is funded by Fern Solutions

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

Filesize dev	Filesize PR	Diff
672.9 kB (166.8 kB)	673.1 kB (166.8 kB)	+183 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

Filesize dev	Filesize PR	Diff
452.6 kB (109.3 kB)	452.8 kB (109.4 kB)	+183 B

[Mugen87]

/ping @cabanier #22595 eventually landed although with a slightly different implementation. The performance improvements are in place though 🎉 .

[cabanier]

Very nice! Do we need the depth buffer data from the transmission pass? If not, you can discard it at the end of the transmission pass of each eye so the GPU doesn't have to resolve it.