Saves 2k of .bss section.

Not used yet.

There are more broken versions of Tahoma out there on various Windows releases,
so we need to add them to our blacklist to avoid broken rendering.
See https://bugzilla.mozilla.org/show_bug.cgi?id=1279925 for details.

Patch from Phil Race.

In https://crbug.com/681813 another instance of Padauk was identified
triggering collapsed glyphs. Blacklist this version by patching
hb-ot-layout.cc to print out gdef, gsub, and gpos table length, then
adding those to the list of blacklisted versions.

Supports enumerating variation axes, normalizing values, etc.

With this, I believe MATH table API is not ready to be merged.

Note, the function now returns "half of horizontal advance width"
if top accent attachment for glyph is not explicitly defined.
This is what the spec requires.  Updated tests.

According to comments in issue #305 broken GDEF tables lead
to incorrect rendering of Latin characters. Blacklisting
this font.

This is based on bug reports that have been filed against Firefox since it
updated to a version of harfbuzz that uses zeroing by GDEF rather than by
Unicode. I'm sure there are a bunch more font versions that should also be
included; these are just the ones I have on hand and have confirmed as having
bad GDEF data.

Given how the list here is growing, I think we should reconsider the approach,
and perhaps revert to zeroing by Unicode instead.

Fixes https://github.com/behdad/harfbuzz/issues/264
Fixes https://github.com/behdad/harfbuzz/pull/266