Add XRRenderTarget.

[Mugen87]

Related issue: #30365

Description

The PR is a follow up of #30365 that introduces XRRenderTarget which is a specific type of render target intended for XR. It allows the engine to store XR specific properties in the render target without polluting/monkey-patching the original RenderTarget scope.

[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	505.56 140.43	505.84 140.49	+278 B +57 B
WebGPU Nodes	505.03 140.32	505.31 140.38	+278 B +56 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	579.6 157.12	579.87 157.17	+278 B +51 B
WebGPU Nodes	534.98 146.67	535.26 146.73	+278 B +58 B