Vulkan: Pre-work for copyImage/copySubImage features - 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>