WebGPURenderer: Make Animation WebXR compatible.

[Mugen87]

Related issue: #28968

Description

This is a first gentle step towards WebXR support in WebGPURenderer. The internal Animation module must be refactored a bit so it can be used in an upcoming XRManager module.

A native XRSession object offers similar to window a requestAnimationFrame() function. When presenting, it must be used instead of the window version. WebXRManager also overwrites the animation loop which is set on app level (via renderer.setAnimationLoop()) which is why the module currently uses an own instance of Animation.

To make this work, Animation requires start() and stop() methods as well as setContext(). setContext() is later used in XRManager with the current XR session object. _init() is replaced with start().

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	692.66 171.56	692.66 171.56	+0 B +0 B
WebGPU	822.12 221.9	822.22 221.93	+105 B +32 B
WebGPU Nodes	821.63 221.78	821.73 221.81	+105 B +30 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.01 112.35	465.01 112.35	+0 B +0 B
WebGPU	542.65 146.78	542.76 146.81	+105 B +26 B
WebGPU Nodes	498.65 136.58	498.76 136.61	+105 B +32 B