XRManager: Add MSAA support.

[Mugen87]

Related issue: #28968

Description

This PR adds MSAA support for the layers code path.

MSAA is implemented with the default code path using renderbuffers which requires an explicit resolve. Support for WEBGL_multisampled_render_to_texture will be added separately.

[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	510.39 141.66	510.54 141.69	+149 B +32 B
WebGPU Nodes	509.86 141.55	510 141.59	+149 B +32 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	584.39 158.38	584.54 158.41	+149 B +29 B
WebGPU Nodes	539.77 147.93	539.92 147.96	+149 B +29 B

[mrdoob]

Interesting that it's the MSAA what breaks the lines in these screenshots.

SwiftShader seems to be kind of deprecated though...