Update harfbuzz to 1.2.5

Overview of changes leading to 1.2.5
Monday, April 4, 2016
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- Fix GDEF mark-filtering-set, which was broken in 1.2.3.

Overview of changes leading to 1.2.4
Thursday, March 17, 2016
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- Synthesize GDEF glyph class for any glyph that does not have one in GDEF.
  I really hope we don't discover broken fonts that shape badly with this
  change.
- Misc build and other minor fixes.
- API changes:
  - Added HB_NDEBUG.  It's fine for production systems to define this to
    disable high-overhead debugging checks.  However, I also reduced the
    overhead of those checks, so it's a non-issue right now.  You can
    forget it.  Just not defining anything at all is fine.

(prlw1)