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

See discussion:
https://lists.freedesktop.org/archives/harfbuzz/2016-February/005489.html

Right now the position_finish_advances() is empty.  To be used for
spacing attachments proposal later.

This speeds up shaping the Amiri font by over 15%.

This was primarily needed for my work on OpenType GX, since
we will be collecting only sublookups that are "active" for
current font instance; but it's a nice boost in general as
well.

We might, in the future, collect subtables in the lookup_accel.
That would also allow us to do a per-subtbale set-digest, which
should speed things up some more, specially for ContextChainFormat3
lookups...  Amiri, for example, contains one lookup with 53
subtables!

To lower the number of gtk-doc warnings.

Currently just announces lookup applications.  Message-API *will* change.
hb-shape / hb-view are updated to print-out messages to stder if --debug
is specified.