This problem was uncovered when I inadvertently changed TextureD3D to
delete its storage before its images. Small mips of compressed
textures must use a nullptr argument to CopySubResource, otherwise
the runtime complains about un-aligned sizes.

Also change the class to delete the Images before the Storage again
so we don't wastefully recover the images before deleting them. Also
change the Image pointers to use std::array and std::unique_ptr.

BUG=angleproject:1156
BUG=angleproject:2077

Change-Id: Idb2e53835b7a9b973285ff0781f70b25f05c77aa
Reviewed-on: https://chromium-review.googlesource.com/543438Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>