WebGPURenderer: multiple canvas support

[aardgoose]

This introduces a CanvasRenderTarget() to represent a screen canvas, which can be used via renderer.setRenderTarget() for use with WebGPU, thus supporting multiple canvases.

The default screen canvas state in Renderer() is replaced by a CanvasRenderTarget(). - this may be too much code change?

[LeviPesin]

The API feels amazing!!

[sunag]

This PR sounds great to PR. @mrdoob @Mugen87 Any thoughts on this approach?

const canvasRT = new CanvasRenderTarget( { domElement } );

renderer.setRenderTarget( canvasRT );

[github-actions[bot]]

📦 Bundle size

Full ESM build, minified and gzipped.

Filesize dev	Filesize PR	Diff
683.5 kB (169.2 kB)	683.5 kB (169.2 kB)	+0 B

🌳 Bundle size after tree-shaking

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

Filesize dev	Filesize PR	Diff
460.7 kB (111.1 kB)	460.7 kB (111.1 kB)	+0 B