WebGLShadowMap: Support alphaToCoverage with shadows.

[Mugen87]

Fixed #30462.

Description

Materials might use alphaToCoverage to implement grass sprites/billboards like mentioned in #30462. WebGLRenderer is currently not able to provide proper shadows with such a setup. alphaToCoverage can only be used in a multisampled context which is not true for shadow map generation. So the idea is to approximate shadows like suggested by #30760. This PR implements a different approach with less changes in WebGLShadowMap. The shared codesandbox in #30760 (comment) is fixed with this change.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.44 78.37	336.49 78.38	+50 B +9 B
WebGPU	541.97 150.12	541.97 150.12	+0 B +0 B
WebGPU Nodes	541.44 150.01	541.44 150.01	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.37 112.21	465.42 112.22	+50 B +11 B
WebGPU	614.81 166.16	614.81 166.16	+0 B +0 B
WebGPU Nodes	569.8 155.56	569.8 155.56	+0 B +0 B

[Mugen87]

I've tried adding this fix for WebGPURenderer as well. However, even materials with just alphaTest are not yet supported (shadows extend uniformly over the entire plane) so more updates are required in WebGPURenderer. Better to do this with a separate PR.

Reproduction test: https://jsfiddle.net/Lt1d32m7/