Function compactWebGPUReadbackRows

Drop WebGPU's per-row buffer padding from a readback typed array.

WebGPU's copyTextureToBuffer (used internally by WebGPURenderer.readRenderTargetPixelsAsync) requires bytesPerRow to be a multiple of 256 (WebGPU spec § "Texture & buffer copy alignment"). Three.js sizes the returned typed array to match the padded GPU layout, so when width * bytesPerTexel isn't already a multiple of 256 the array contains junk bytes at the end of every row.

This helper returns a row-compacted view sized exactly width * height * bytesPerTexel. When the input already matches the compact size (no padding required), the input is returned unchanged to avoid an allocation. Both WebGL2-returned arrays (always compact) and WebGPU-returned arrays for aligned widths hit the no-op path.

Same math as the PickingSystem.readbackAndVote deinterlace loop; extracted so the post-processing capture/screenshot paths can share it (previously they assumed compact rows under WebGPU and produced corrupted output for canvas widths whose row stride wasn't a multiple of 256 bytes).