This removes the coverage of non-dedicated allocations, but we've never
supported that properly anyway (we require that the offset passed to
glTexStorageMem2DEXT be zero).

Bug: angleproject:4627
Change-Id: I14b0f39e5e13c3aafc4549dc59b6e3ac4ec6ad8c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2192500Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>