WebGPU: Use the correct source format for ReadPixels ReadPixels was using the format of the destination buffer which works correctly for RGBA8 -> RGBA/UNSIGNED_BYTE but runs the wrong conversion functions for other formats. Bug: angleproject:356399840 Bug: angleproject:42267090 Change-Id: I7c46add3f643017f454136135afef697847a2383 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5748446 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>