WebGPURenderer: Fix premultiplied alpha with clear colors.

[Mugen87]

Fixed #29337

Description

This should fix the clear color differences mentioned in #29337.

There was a bug that premultiplied alpha twice for the WebGL backend. The policy should be:

• When the clear color from the render context is used, it already has premultiplied alpha added (Background takes care of that).
• When the original clear color is queried via getClearColor(), alpha has not been premultiplied so far. That must be done if it is used for the actual clear operation.
• Premultiplied alpha clear colors must always be used with the WebGL backend since it always uses alpha: true for the WebGL rendering context. With WebGPU, premultiplied alpha clear colors are only required when setting alpha: true when creating the renderer.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	688.13 170.5	688.13 170.5	+0 B +0 B
WebGPU	807.15 217.43	807.26 217.48	+109 B +49 B
WebGPU Nodes	806.66 217.29	806.77 217.34	+109 B +50 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	462.85 111.78	462.85 111.78	+0 B +0 B
WebGPU	535.95 144.69	536.06 144.73	+109 B +39 B
WebGPU Nodes	492.03 134.42	492.14 134.46	+109 B +37 B

[WestLangley]

In my testing, it seems that both backends render the same with this PR, but the WebGL backend no longer matches WebGLRenderer. Tone mapping disabled for comparison.

#29542 is limiting my ability to fully test this. Proper compositor blending is most easily verified in linear color space.

[Mugen87]

There is definitely a difference when comparing WebGLRenderer with WebGPURenderer since the example uses blending and that is only fully correct in WebGPURenderer. When using sRGB output, WebGLRenderer performs blending in the wrong color space due to the inline encode.

[Mugen87]

I'm going ahead and merge since the WebGL fallback code path is definitely wrong right now.