[HarfBuzz] 'kern' feature in Indic scripts

[Jonathan Kew]

In override_features_indic(), we currently disable 'kern' in 
uniscribe-compatible mode. However, at least for the Noto Telugu font on 
Win2012Server, I'm seeing that 'kern' *is* applied by Uniscribe. As a 
result, running tests with it disabled is leading to a lot of unwanted 
positioning discrepancies.

So I think we need to look into this again; offhand, I don't recall what 
tests led us to conclude it wasn't applied. Or maybe it's a recent 
(Win8?) change in Uniscribe behavior...? (I haven't tried to re-test on 
older Windows versions yet.)

JK