Vulkan: Implement cube map render targets. Each TextureVk now stores vectors of RenderTargetVks and ImageViews associated with the image faces. They are initialized lazily when the RenderTarget is queried in getAttachmentRenderTarget. There's still one missing edge case for handling clear with the Framebuffer when using cube maps. Also one additional test failure on Android. Bug: angleproject:2470 Change-Id: Ib959a3434a992cef010a11940cf2ee49e118ac17 Reviewed-on: https://chromium-review.googlesource.com/1220727 Reviewed-by: Frank Henigman <fjhenigman@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>