* [aat] Fix implementation of AAT kerning for Geeza Pro.
Despite what the comment in the code used to say, it appears that Geeza Pro
does rely on accumulating kerning values from successive subtables. With
this change, the results now match Core Text rendering (and avoid the clear
visual breakage reported in #2358).
Testcase: U+0644,U+064E,U+0645,U+064E,U+0651,U+0627
Fixes#2358.
* [aat] Update test expectations, add new testcase.