Use const pointers to the statically allocated structures instead of
copying them in TextureStorage11. This avoids the cost of copying and
saves a little bit of memory.

BUG=angleproject:1244
TEST=angle_end2end_tests

Change-Id: Ib59fddd68ba9bc53e491d55683416c0661f26e0e
Reviewed-on: https://chromium-review.googlesource.com/333930Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>