Similarly to how the subImage calls were corrected. glTexImage and the
family of related functions apply to a single texture target. This
means we need a different TexImage call for each cube face. This is
distinct from TexStorage calls which take a Cube map directly.

Note this is mostly a refactoring change. But it does increase code
consistency and should allow for more efficient code reuse.

Bug: angleproject:3356
Change-Id: I252f8983cdda3a8f0223c44cbbe8d8e2dd319a88
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1558673
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>