PMREMGenerator: Fix compile methods and background usage.

[Mugen87]

Related issue: -

Description

App code might use this pattern to configure the background:

const pmremGenerator = new THREE.PMREMGenerator( renderer );
pmremGenerator.compileEquirectangularShader();

const renderTarget = pmremGenerator.fromEquirectangular( texture );
scene.background = renderTarget.texture;

The PR fixes the PMREMGenerator.compile*() methods by making sure they are not using an undefined mesh for WebGPURenderer.compile(). Besides, it makes sure Scene.background accepts env maps with CubeUVReflectionMapping.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	685.6 kB 169.8 kB	685.6 kB 169.8 kB	+0 B +0 B
WebGPU	821.7 kB 220.7 kB	821.8 kB 220.7 kB	+63 B +15 B
WebGPU Nodes	821.3 kB 220.6 kB	821.3 kB 220.6 kB	+482 B +109 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	462.2 kB 111.5 kB	462.2 kB 111.5 kB	+0 B +0 B
WebGPU	522.1 kB 140.6 kB	522.2 kB 140.7 kB	+67 B +24 B
WebGPU Nodes	478.8 kB 130.5 kB	478.8 kB 130.5 kB	-43.29 kB +22 B