Reset the base level of textures before call glTexImage on Mac
to prevent driver bugs that cause texture corruption.

BUG=angleproject:3671

Change-Id: I90e94b8395a781a2142ef6be3af1d6117f084152
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1692975
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>