Cleared confusion between GL level indices and VK level indices by
adding the corresponding suffix to variables and function arguments.  A
handful of places that sent one index and expected the other are fixed.

The conversion between the two is given by:

    levelIndexGL = levelIndexVk + baseLevel;

Bug: angleproject:4695
Change-Id: I84ecbaf867d00a40fb39b6db7ad79658016f4d9a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2235362
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>