PMREMGenerator: Apply await renderer.init() policy.

[Mugen87]

Related issue: -

Description

The PR makes sure PMREMGenerator honors the await renderer.init() policy like other modules. Meaning certain method are only allowed to be used after the renderer has been initialized.

The internal fallback usage of *Async() methods is obsolete since they are already deprecated and will be removed in a couple of releases.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	365.32 86.78	365.32 86.78	+0 B +0 B
WebGPU	660.7 182.49	660.5 182.46	-203 B -36 B
WebGPU Nodes	658.82 182.21	658.62 182.17	-203 B -38 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	498.04 121.5	498.04 121.5	+0 B +0 B
WebGPU	732.57 197.4	732.37 197.35	-206 B -44 B
WebGPU Nodes	681.79 184.78	681.59 184.75	-206 B -33 B