Vulkan: Support EGL images sourced from cube maps. Store an image array offset and source texture type in ImageVk to select the correct cube face from the source texture. BUG=angleproject:2668 Change-Id: I03ad25feccb769c906dd28fb573ec342e7816863 Reviewed-on: https://chromium-review.googlesource.com/c/1422542 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>