WebGLBackground: Reset meshes in dispose().

[Mugen87]

Fixed #30230.

Description

When renderer.dispose() is called, the background's dispose() method is called as well since r171. If a renderer is used after renderer.dispose() which is true for the editor, the background wasn't functional anymore. To force a call of WebGLObjects.update(), WebGLBackground.dispose() simplify removes the references to its internal meshes which is also the more correct implementation.

It is open for debate if using a renderer after dispose() is valid but this PR restores the behavior before r171.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	339.47 79.08	339.49 79.09	+18 B +5 B
WebGPU	489.82 135.88	489.82 135.88	+0 B +0 B
WebGPU Nodes	489.29 135.76	489.29 135.76	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.33 112.13	465.35 112.14	+18 B +7 B
WebGPU	559.3 151.37	559.3 151.37	+0 B +0 B
WebGPU Nodes	515.38 141.15	515.38 141.15	+0 B +0 B