WebGPURenderer: Fallback for alphaToCoverage if antialias is disabled

[RenaudRohlinger]

Related issue: #29269

Description

Safeguarded the renderer from breaking when alphaToCoverage is used without multisampling. For example InstancedPointsNodeMaterial would break by default if the renderer didn't have antialias enabled.

This contribution is funded by Utsubo

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	685.83 169.75	685.83 169.75	+0 B +0 B
WebGPU	832.14 222.98	832.25 223	+115 B +18 B
WebGPU Nodes	831.65 222.86	831.76 222.88	+115 B +18 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	462.42 111.53	462.42 111.53	+0 B +0 B
WebGPU	529.9 142.75	529.97 142.77	+70 B +16 B
WebGPU Nodes	486.56 132.61	486.63 132.63	+70 B +15 B

[aardgoose]

Line2NodeMaterial will also need this.

[RenaudRohlinger]

Indeed! Updated the PR consequently.