CL/VK: Add readBufRect staging/transfer routine Adding enqueueReadBufferRect staging/transfer code/routine for the non-blocking conditions. Allows us to correctly stage writes in a batch of cmds with multiple enqueueReadBufferRect cmds. Bug: angleproject:42267074 Change-Id: I46b7b9f4e1fbcd0314a7700525935ed2743b1587 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6068287 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Cody Northrop <cnorthrop@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>