WebGPURenderer: Honor Texture.premultiplyAlpha in WebGPU backend.

[Mugen87]

Related issue: #31132

Description

The PR makes sure Texture.premultiplyAlpha is honored in the WebGPU backend for all image based textures. That aligns the behavior to the WebGL backend which already supported the flag similar to WebGLRenderer.

The PR also removes an unused method in WebGPUTextureUtils.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	337.26 78.64	337.26 78.64	+0 B +0 B
WebGPU	551.04 152.81	551.02 152.82	-12 B +8 B
WebGPU Nodes	550.39 152.66	550.38 152.66	-12 B +8 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	468.44 113.28	468.44 113.28	+0 B +0 B
WebGPU	626.05 169.5	626.03 169.47	-20 B -22 B
WebGPU Nodes	580.9 158.82	580.88 158.81	-20 B -16 B