Different levels of the same texture may have different formats, so
changing the base level may affect the format that should be used for
the storage. Take this into account in the D3D backend.

The added test fails on some GL drivers.

TEST=angle_end2end_tests
BUG=angleproject:596

Change-Id: I5380e942694a75685ebb510edb01c0489e0d5179
Reviewed-on: https://chromium-review.googlesource.com/344230Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>