WebGPURenderer: Add setOutputRenderTarget()

[sunag]

Related issue: #30387 (comment)

Description

The PR introduced Renderer.setOutputRenderTarget() which is focused on screen output, making it less confusing to use RenderTargets and initial support for PostProcessing in XRManager.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	336.12 78.22	336.12 78.22	+0 B +0 B
WebGPU	516.99 143.53	517.38 143.74	+389 B +209 B
WebGPU Nodes	516.45 143.43	516.84 143.63	+389 B +201 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.17 112.09	465.17 112.09	+0 B +0 B
WebGPU	588.99 159.73	589.27 159.82	+285 B +92 B
WebGPU Nodes	544.35 149.29	544.64 149.38	+285 B +90 B

[mrdoob]

Should we call it setOutputRenderTarget()?

[Mugen87]

If the parameter is always a render target, I would adhere to the set*RenderTarget() syntax.

To clarify the name setXRTarget(): I've not use setXRRenderTarget() since the parameter is not a render target but a plain framebuffer in WebGL (it's not yet clear to me how the XR target looks like in WebGPU).