WebGPURenderer: Introduce initRenderTarget().

[Mugen87]

Related issue: -

Description

TRAA and other code sometimes requires to copy render target data. Render targets might be in a state though where they are not initialized yet e.g. right after a dispose() due to a resize.

Previously we initialized the render targets by clearing them once but that was just a workaround which introduced unnecessary clears. The PR introduces initRenderTarget() (similar to WebGLRenderer.initRenderTarget()) that only performs the needed operations meaning setting up correct render context data and for the WebGL backend configure the framebuffer state.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	357.49 84.84	357.49 84.84	+0 B +0 B
WebGPU	622.1 172.81	622.71 172.91	+607 B +103 B
WebGPU Nodes	620.7 172.57	621.31 172.67	+607 B +104 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	490.47 119.9	490.47 119.9	+0 B +0 B
WebGPU	693.21 188.1	693.82 188.21	+607 B +103 B
WebGPU Nodes	642.59 175.2	643.2 175.31	+607 B +104 B

[Mugen87]

I'll go ahead and merge since I need initRenderTarget() on dev to investigate another bug.