- Add vkCmdCopyImageToBuffer support in CommandBuffer
- Add a new function in the PixelBuffer to support staging changes
coming from an image.

Bug: angleproject:2500
Bug: angleproject:2501

Change-Id: Ic29101391edc6f25a71dcd318bc8a72b2ae1a054
Reviewed-on: https://chromium-review.googlesource.com/1054104
Commit-Queue: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>