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/1220727Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>