Vulkan: Make RenderTargetVk use ImageHelper. Bug: angleproject:2318 Change-Id: I9bc4bb7f5bcd3029a31c8570809f253cf5e4b12a Reviewed-on: https://chromium-review.googlesource.com/980773 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Luc Ferron <lucferron@chromium.org> Reviewed-by: Frank Henigman <fjhenigman@chromium.org>