Vulkan: Support cube mimaps generation Bug: angleproject:2502 Change-Id: I953d99d04608cec04aad824b8b38f388ed1e4c2b Reviewed-on: https://chromium-review.googlesource.com/1069544 Commit-Queue: Luc Ferron <lucferron@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>