[HarfBuzz] tibetan marks

[Jonathan Kew]

Turns out that zeroing marks by GDEF in Tibetan (commit 
d5bd0590ae2fbc7b0dee86385a565aef00ffb835) has significantly regressed 
our numbers for the Windows himalaya.ttf font. :(

The main problem seems to be the handling of

   0F3C;TIBETAN MARK ANG KHANG GYON;Ps;0;ON;;;;;Y;TIBETAN LEFT BRACE;;;;
   0F3D;TIBETAN MARK ANG KHANG GYAS;Pe;0;ON;;;;;Y;TIBETAN RIGHT BRACE;;;;

both of which the GDEF table in himalaya.ttf classifies (incorrectly) as 
class 3. Result:

http://ec2-54-226-13-158.compute-1.amazonaws.com/testcase-view.html?file=out/fonts/win8/himalaya.ttf/texts/tibt/wikipedia/bo.txt.diffs#P_1_1_77

and lots of similar examples where these characters are used.

Sigh.

JK