WebGPURenderer: ReadRenderTargetPixelsAsync (RFC)

[aardgoose]

The WebGPU renderer doesn't have a readRenderTargetPixels or equivalent api yet. WebGPU readback is async by nature so another API is required, this is a plain equivalent returning a Promise that resolves to a TypedArrayBuffer allocated by WebGPU, rather than the user providing an array to receive the results.

Open issues:

• Previous discussions of an appropriate async api for WebGL2 rendering, see WebGLRenderer: Async Readback API - WIP #24466 etc.
• The requirement that the bytesPerRow for the GPUBuffer layout must be a multiple of 256.
• The returned TypedArrayBuffer is not 'transferable', a restriction that does not apply to user supplied buffers.

@sunag

[sunag]

@aardgoose Great, I just don't merge it now because the @Mugen87 review is very important for this PR :)