WebGLBackend: Fix clear() with a disabled color write mask.

[drakeo338]

Related issue: #34728

Description

Fixes #34728. On the WebGL2 backend of WebGPURenderer, WebGLBackend.clear() re-enabled the depth and stencil write masks but not the color one, so after a draw with colorWrite: false the next color clear was masked out and the render target kept its old content. The change calls setColorMask( true ) before clearing color, and adds a regression test that fails without it.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
Core	390.08 102.3	390.08 102.3	+0 B +0 B
WebGL	372.73 87.14	372.73 87.14	+0 B +0 B
WebGPU	428.77 115.55	428.8 115.55	+31 B +6 B
WebGPU Nodes	716.44 196.86	716.47 196.86	+31 B +5 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	517.46 124.27	517.46 124.27	+0 B +0 B
WebGPU	769.05 206.07	769.08 206.08	+31 B +5 B
WebGPU Nodes	714.9 192.6	714.93 192.61	+31 B +7 B