Instead of simply returning a count of all non-empty texture
desc levels, we iterate the levels from the base level and return
the count of levels that make a consistent mip chain. This will
fix edge cases where Textures are defined sequentially with
inconsistent mip levels, but valid rendering states.

Bug: angleproject:4780
Change-Id: Id3ebf2d7ad706070896cb5cdf2a311dba2c88949
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3007135Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>