WebGPURenderer: Fix scissor value bound check

[sunag]

Fixes #31777

Description

Fix scissor value bound check, supporting RenderTarget and fixed RenderContext.(width/height) which could have an uninitialized value.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.91 79.11	338.91 79.11	+0 B +0 B
WebGPU	579.36 159.65	579.35 159.66	-6 B +9 B
WebGPU Nodes	577.97 159.4	577.96 159.41	-6 B +10 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	470.78 113.9	470.78 113.9	+0 B +0 B
WebGPU	649.51 175.52	649.5 175.53	-6 B +9 B
WebGPU Nodes	603.6 164.66	603.6 164.67	-6 B +9 B