PMREMGenerator: Fix usage with PMREMNode.

[Mugen87]

Related issue: -

Description

Two or more PMREM nodes might refer to the same source textures. This use case currently works just for the first PMREM node since the second one produce a runtime error that can be reproduce with webgpu_materials_envmaps_bpcem. Just unselect the checkbox "box projected" and check the browser console:

three.webgpu.js:22545 Uncaught TypeError: Cannot set properties of undefined (setting 'material')
at PMREMGenerator._textureToCubeUV (three.webgpu.js:22545:17)
at PMREMGenerator._fromTexture (three.webgpu.js:22362:8)
at PMREMGenerator.fromCubemap (three.webgpu.js:22232:15)
at _getPMREMFromTexture (three.webgpu.js:22919:30)
at Proxy.updateBefore (three.webgpu.js:23140:13)
at Proxy.setup (three.webgpu.js:23164:8)
at Proxy.build (three.webgpu.js:1678:29)
at Proxy.build (three.webgpu.js:2209:16)
at Proxy.setup (three.webgpu.js:7086:26)
at Proxy.build (three.webgpu.js:1678:29)

That happens because every PMREM node has its own PMREMGenerator. The render targets used for generating the PMREM are cached and managed globally for the source texture. Meaning if the second generator wants to re-generate the PMREM, its internal state isn't ready. The PR fixes this issue by renaming _allocateTargets() to _allocateTarget() and just use the method for allocating the destination render target. A new _init() method makes sure internal resources (e.g the LOD planes or the blur material) are created if not ready. Exactly these resource were missing in the second generator and produced the above runtime error.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.33 78.33	336.33 78.33	+0 B +0 B
WebGPU	547.45 151.77	547.49 151.78	+43 B +9 B
WebGPU Nodes	546.8 151.61	546.84 151.62	+43 B +8 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.78 112.28	465.78 112.28	+0 B +0 B
WebGPU	622.3 168.35	622.34 168.36	+43 B +9 B
WebGPU Nodes	577.17 157.62	577.22 157.63	+43 B +3 B