Material: Set premultipliedAlpha true by default

[mrdoob]

Related issue: #32449 (comment) #32470

On a related topic, I'm wondering if it's worth raising the idea of setting the default of "premultipliedAlpha" to true, again, since the results are generally much nicer for transparent objects in bright lighting.

[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 +16 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

[mrdoob]

Seems like too many things break if we do this:
https://github.com/mrdoob/three.js/actions/runs/19917173934?pr=32472

[gkjohnson]

Yeah I think it will make everything with transparency break 😅. Something to maybe tackle more slowly.

[mrdoob]

Closing for now.