This CL stores the d3d11::TextureFormat by reference in the texture
storage. Adding the internalFormat to the TextureFormat allows us
to store a single ref instead of three per TextureStorage11.

Also store the format sets in a d3d11::TextureFormat by-ref instead
of by-pointer, making the code a bit cleaner.

BUG=angleproject:1455

Change-Id: I3c0e966d948c694435577d7d45dc0cd156480cdb
Reviewed-on: https://chromium-review.googlesource.com/365412Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>