fix up AA for WebXR and use multisample extension only when needed

[cabanier]

This diff brings back multisampling for WebXR when using projection layers.
It also disables multisampled_render_to_texture unless the rendertarget was set up with explicit textures.
It also clears the render target when exiting WebXR. Previously, the XR render target continued to be used which resulted in errors.

This contribution is funded by Meta

cc @Mugen87

[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	520.64 145.15	520.73 145.18	+81 B +21 B
WebGPU Nodes	520.11 145.05	520.19 145.07	+81 B +22 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	591.03 160.79	591.12 160.82	+81 B +27 B
WebGPU Nodes	546.16 150.23	546.24 150.25	+81 B +24 B