XRManager: Fix background.

[Mugen87]

Related issue: #30346

Description

Small follow-up to fix backgrounds in XR.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.25 78.28	336.25 78.28	+0 B +0 B
WebGPU	503.11 139.72	503.18 139.76	+68 B +39 B
WebGPU Nodes	502.58 139.61	502.65 139.65	+68 B +39 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.25 112.13	465.25 112.13	+0 B +0 B
WebGPU	576.89 156.36	576.96 156.41	+68 B +44 B
WebGPU Nodes	532.28 145.92	532.35 145.96	+68 B +43 B