WebGPURenderer: Harmonize clear of MRT textures.

[Mugen87]

Fixed #30567.

Description

The PR harmonizes how MRT render targets are cleared in WebGPURenderer.

Until it is possible to define clear values for each color attachment, the clear color is now only applied to the beauty attachment (the default color attachment). All others are using 0, 0, 0, 1 which is already the default in the WebGPU backend when a clear operation happens in beginRender().

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.65 78.39	336.65 78.39	+0 B +0 B
WebGPU	522.23 144.94	522.37 144.98	+134 B +37 B
WebGPU Nodes	521.7 144.84	521.83 144.88	+134 B +41 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.64 112.24	465.64 112.24	+0 B +0 B
WebGPU	592.61 160.56	592.74 160.6	+134 B +37 B
WebGPU Nodes	547.74 150	547.87 150.04	+134 B +36 B

[Mugen87]

Now the webgpu_multiple_rendertargets* examples are rendered consistently across WebGPU and WebGL 🎉 .