When vkCmdCopyImageToBuffer is possible, it is used to implement
glReadPixels with PBO instead of a CPU readback.

A few tests are added to verify synchronization between pack/unpack and
compute shader writes.

Bug: angleproject:5070
Change-Id: Ia5da0feb31348f6b52d164b973ff95e2a0510582
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2691466
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Ian Elliott <ianelliott@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>