WebGPURenderer: Fix material transparent assignment

[sunag]

Fixes #31768 (comment)

Description

I moved the backdrop code to the rest of the code. It should be useful for custom refraction effects.

Thanks to @Mugen87 and those involved for finding the cause of the problem. 🙏

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	350.11 84.81	350.11 84.81	+0 B +0 B
WebGPU	604.15 169.43	604.45 169.49	+301 B +63 B
WebGPU Nodes	602.75 169.19	603.06 169.25	+305 B +60 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	481.77 119.56	481.77 119.56	+0 B +0 B
WebGPU	673.57 184.91	673.88 184.97	+311 B +62 B
WebGPU Nodes	615.56 168.12	615.87 168.19	+311 B +64 B