PMREMGenerator: Add optional renderTarget in fromScene()

[sunag]

Related issue: #29951, #29942

Description

This PR improves backwards compatibility, even if the device is not initialized it returns the texture it will later use to generate the PMREM but keep the warnings.

• Add optional renderTarget in fromScene()
• Return the RenderTarget if asynchronous calls necessary instead of Promise.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	339.14 78.99	339.14 78.99	+0 B +0 B
WebGPU	483.15 134.15	483.41 134.17	+260 B +17 B
WebGPU Nodes	482.62 134.06	482.88 134.07	+260 B +15 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	464.59 111.96	464.59 111.96	+0 B +0 B
WebGPU	552.25 149.49	552.51 149.51	+260 B +26 B
WebGPU Nodes	508.13 139.2	508.39 139.23	+260 B +24 B