WebGLBackend: Configure scissor/viewport before clear.

[Mugen87]

Fixed #30450.

Description

The WebGL backend of WebGPURenderer must configure scissor and viewport before a clear operation. Otherwise it uses outdated values from the previous render.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.12 78.22	336.12 78.22	+0 B +0 B
WebGPU	517.05 143.52	517.05 143.52	+0 B -1 B
WebGPU Nodes	516.52 143.42	516.52 143.41	+0 B -2 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.17 112.09	465.17 112.09	+0 B +0 B
WebGPU	589.05 159.72	589.05 159.72	+0 B -1 B
WebGPU Nodes	544.42 149.28	544.42 149.28	+0 B +0 B