Vulkan: Add RenderTargetVk::getImageForRead. This helper method will also transition the Image to the correct read layout. We will need to revisit the implementation when working on simulatenous read. Bug: angleproject:2539 Change-Id: Id61404460f3ef0dbb054e6ac2dfc0b59adb78402 Reviewed-on: https://chromium-review.googlesource.com/1108378 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Luc Ferron <lucferron@chromium.org>