ProjectorLightNode: Fix back-projection.

[Mugen87]

Fixed #31461.

Description

The PR prevents back-projection when using ProjectorLight. This bug was present in the initial code from #24589 and unfortunately ported to the node system.

The idea is to the check the w coordinate of spotLightCoord since this variable represents the fragment's position in the light's clip space. The sign of w determines whether a fragment is in front or behind the light.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	337.92 78.83	337.92 78.83	+0 B +0 B
WebGPU	560.13 155.06	560.22 155.11	+89 B +50 B
WebGPU Nodes	559.06 154.84	559.15 154.89	+89 B +50 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	469.19 113.49	469.19 113.49	+0 B +0 B
WebGPU	635.3 171.98	635.39 172.03	+89 B +45 B
WebGPU Nodes	589.95 161.23	590.04 161.27	+89 B +44 B

[ycw]

Thanks.