Renderer: Disallow render target resizing while in XR.

[cabanier]

When you bring up the browser in overlay while in WebXR on the Quest browser, the example page tries to do a resize.
This messes up the rendertarget because it doesn't expect that its backing can be resized.

I added code to disallow resizing while in XR mode.

This contribution is funded by Meta

[cabanier]

Should setPixelRatio also be gated?

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.02 78.26	336.02 78.26	+0 B +0 B
WebGPU	524.9 146.21	524.97 146.22	+66 B +10 B
WebGPU Nodes	524.37 146.1	524.43 146.12	+66 B +11 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.06 112.14	465.06 112.14	+0 B +0 B
WebGPU	597.68 162.47	597.75 162.49	+66 B +13 B
WebGPU Nodes	552.81 151.96	552.87 151.97	+66 B +12 B

[Mugen87]

Should setPixelRatio also be gated?

I would say no since this method is usually called once when setting up the renderer. Besides, setting the pixel ratio automatically triggers setSize() so if the latter method is gated, everything should be fine.