fontselect: coretext: allow to match family using the provider

Not all APIs cache everything the same way that fontconfig does. This allows
to first perform a match based on the font family and then score the matched
fonts using the common code. The benefit is the application doesn't have to
load all of the fonts and query for weight, slant, width, path and fullnames.

I left both code paths inside ass_coretext.c. This allows to test matching
problems and have a term of comparison with the slower implementation.
To activate it one just has to flip the CT_FONTS_EAGER_LOAD define to 1.

Here some benchmarks with a pretty typical OS X font library of ~600 fonts:

CT_FONTS_EAGER_LOAD=0
time sh ~/dev/test-libass.sh  0.04s user 0.02s system 79% cpu 0.081 total

CT_FONTS_EAGER_LOAD=1
time sh ~/dev/test-libass.sh  0.12s user 0.06s system 44% cpu 0.420 total