Vulkan: Shader path for framebuffer-to-texture copy Part 1 in a series of changes to perform image copies on the GPU. Bug: angleproject:2958 Change-Id: I6264a880865c4738c0866f2dc71af63425fc4118 Reviewed-on: https://chromium-review.googlesource.com/c/1370724 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>