WebGPURenderer: Fix shadow's alphaTest if used material.map

[sunag]

Fixes #31802

Description

Fix shadow's alphaTest if used material.map.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.91 79.11	338.91 79.11	+0 B +0 B
WebGPU	579.29 159.67	579.49 159.77	+198 B +100 B
WebGPU Nodes	577.9 159.43	578.1 159.53	+198 B +99 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	470.78 113.9	470.78 113.9	+0 B +0 B
WebGPU	649.44 175.54	649.64 175.64	+198 B +100 B
WebGPU Nodes	603.54 164.69	603.74 164.79	+198 B +99 B