WebGLRenderer: Apply premultiplied alpha in sRGB space

[mrdoob]

Related issue: #32449 (comment)

After thinking through it for a few days I think I would probably take the approach of converting to the target color space, applying alpha, and converting back to the working color space. This should be not so expensive in the grand scheme of things and only need to be run when premultiplied alpha is true and blending is enabled.

Before	After

It sure looks like an improvement!

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	350.26 83.01	350.03 83.02	-233 B +16 B
WebGPU	616.03 171	616.03 171	+0 B +0 B
WebGPU Nodes	614.64 170.74	614.64 170.74	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	482.3 117.81	482.07 117.82	-233 B +15 B
WebGPU	687.42 186.76	687.42 186.76	+0 B +0 B
WebGPU Nodes	637.26 173.94	637.26 173.94	+0 B +0 B

[gkjohnson]

To me this seems like a good, well targeted change. The last thing I would suggest is not deleting the "premultiplied_alpha_fragment" shader chunk because it will be a breaking change for anyone using it. And not all shaders use the "opaque_fragment" chunk, as you'll see in the LDrawConditionalLineMaterial.js‎ file. I often add it to the end of my shaders after setting the final gl_FragColor value in order to respect the premultiply alpha flag.