Fix: WebGL viewport / scissors not working correctly with PostProcessing in WebGPURenderer

[TobiasNoell]

Fixed #32882

Description

When using PostProcessing with the WebGL backend (forceWebGL: true), viewport and scissor settings were incorrectly applied during nested render calls, causing rendered content to appear smaller than expected.

Root cause: In WebGL, scissor state is global. When PassNode renders the scene to a render target (triggering a nested renderer.render() call), the beginRender method didn't reset the scissor when renderContext.scissor was false. This left the outer scissor active, causing the render target to only receive content within the outer scissor area.

Fix in WebGLBackend.js:

In beginRender: Added else clause to reset scissor to the full render area when renderContext.scissor is false
In finishRender: Added scissor state restoration (matching the existing viewport restoration pattern)
This ensures proper scissor state isolation between nested render calls, matching WebGPU's per-render-pass state model.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	359.21 85.29	359.21 85.29	+0 B +0 B
WebGPU	622.99 173.1	623.23 173.14	+242 B +34 B
WebGPU Nodes	621.57 172.85	621.81 172.89	+242 B +34 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	490.47 119.49	490.47 119.49	+0 B +0 B
WebGPU	689.65 186.27	689.89 186.3	+242 B +34 B
WebGPU Nodes	639.03 173.69	639.27 173.72	+242 B +35 B

[Mugen87]

I think scissor should be managed like viewport so this change looks good to me.