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>