Some Mac GL drivers fail to generate mipmaps if level zero is not set
before the texture is first used, all mipmap data is black.

To work around this, whenever a texture level is allocated, ensure that
level zero is also allocated with at least a 1x1 image.

Bug: angleproject:5223
Change-Id: If1a728e017dec600c77a54f7ae185b719aaaae84
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2497569Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>