fontselect: implement a coretext font provider

fontselect: implement a coretext font provider

Fontconfig is known to be very slow on OS X and Windows, this has to do with
the extremely prohibitive cache times (which are getting even longer with
latest versions of Fontconfig).

This commits starts to address the problem by using CoreText on OS X to load
the font data. The commit uses the simplest possible approach to load all of
the data in memory and then use it to match. This causes a somewhat slow
startup time (around ~400ms on my i7) but it is already better than waiting
*minutes* for Fontconfig to cache the fonts data.

A later commit will improve the speed of the match by using a hybrid approach
that lazy loads in the libass database only the necessary fonts.