This was previously returning maxCombinedTextureImageUnits instead of
maxCombinedUniformBlocks.  Fixing that exposed a bug in the GL backend
where the combined values were sometimes capped to vertex+fragment
values and sometimes not capped at all.

The reasoning for such capping was that the combined limits as queried
from GL contains stages not available in GLES (such as tessellation).
However, the capping failed to take geometry shader limits into account.

This change adjusts such capping to vertex+fragment+geometry.  It also
applies the cappping to all combined limits, rather than a select few.

Bug: angleproject:2099
Change-Id: I7231058b5d7f80b1b2452d9f87d4b0ab6e1cdb17
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1572487Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>