XRManager: Request reference space for both layers and non layers.

[danrossi]

Fixed #30901

Description

When using the WebGPURenderer and forcing WebGL for WebXR. XRManager isn't setting up the reference space for non layers.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.49 78.38	336.49 78.38	+0 B +0 B
WebGPU	542.05 150.13	542.05 150.13	+0 B -1 B
WebGPU Nodes	541.51 150.03	541.51 150.03	+0 B -1 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.42 112.22	465.42 112.22	+0 B +0 B
WebGPU	614.89 166.17	614.89 166.17	+0 B +0 B
WebGPU Nodes	569.88 155.58	569.88 155.58	+0 B -1 B

[danrossi]

Looks good thanks.