NodeManager: Correctly dispose of background mesh.

[Mugen87]

Fixed #34103.

Description

When a texture is assigned to Scene.background, the renderer creates an internal node. This node has a dispose() routine but it only ever executes if the node is maintained on app level via Scene.backgroundNode and node.dispose() is called manually.

When the app just assigns a texture and calls dispose(), the above node is never removed. Since there is no Scene.dispose(), the only way to get rid of internal resources is to couple the texture disposal with the background node disposal. The PR implements that in NodeManager.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	365.88 86.71	365.88 86.71	+0 B +0 B
WebGPU	683.3 189.04	683.47 189.09	+169 B +46 B
WebGPU Nodes	681.29 188.72	681.46 188.76	+169 B +46 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	510.55 124.02	510.55 124.02	+0 B +0 B
WebGPU	761.67 205.09	761.84 205.14	+169 B +46 B
WebGPU Nodes	711.17 192.51	711.34 192.56	+169 B +45 B