[WebGPU] Implement texture copy from cpu reads. This CL implements a slow path to copying from a texture that reads data from the CPU. This path is taken when the src and destination formats differ. Bug: angleproject:438268609 Change-Id: I5a81fbed5bbac68c48537d8c7a2fcc487c31e76e Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6847147 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Matthew Denton <mpdenton@chromium.org> Auto-Submit: Liza Burakova <liza@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>