WebGPURenderer: Set opaque blend mode for secondary MRT targets

[sunag]

Related issue: #32252 (comment)

Description

The PR changes the MRT pipeline configuration so that only the first render target texture (index 0) uses the material's blending mode, while all additional textures operate in opaque mode (no blending). This is a common pattern for MRT setups where auxiliary G-buffers (normals, depth, velocity, etc.) should write data directly without blending.

I think it's best if we use this standard until we add these features to the API.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	356.24 86.48	356.24 86.48	+0 B +0 B
WebGPU	615.15 172.62	615.49 172.75	+337 B +127 B
WebGPU Nodes	613.76 172.37	614.1 172.5	+337 B +126 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	488.04 121.25	488.04 121.25	+0 B +0 B
WebGPU	684.62 187.89	684.96 188.02	+337 B +132 B
WebGPU Nodes	626.34 171.11	626.68 171.22	+337 B +115 B