PassNode: Fix scissor and viewport setup.

[Mugen87]

Fixed #33203.

Description

The scene setup in #33203 isn't valid but even with the intended approach it turns out the scissor and viewport dimensions are not correctly computed in PassNode. The pixel ratio might not be correct when setScissor() and setViewport() are called so it's important to compute the effective dimensions in setSize().

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	359.97 85.54	359.97 85.54	+0 B +0 B
WebGPU	632.09 175.38	632.12 175.39	+38 B +6 B
WebGPU Nodes	630.2 175.09	630.24 175.1	+38 B +7 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	492.17 120.12	492.17 120.12	+0 B +0 B
WebGPU	704.52 190.19	704.55 190.19	+38 B +8 B
WebGPU Nodes	653.74 177.59	653.78 177.6	+38 B +6 B