Instead of loading glyphs (with FreeType), just check loca table

Part of https://bugs.freedesktop.org/show_bug.cgi?id=64766#c47

This is the approach introduced in
https://bugs.freedesktop.org/show_bug.cgi?id=64766#c30

Testing it with 11GB worth of stuff, before/after:

behdad:src 130$ time fc-scan ~/fonts/ > before

real	2m18.428s
user	1m17.008s
sys	0m20.576s

behdad:src 0$ time fc-scan ~/fonts/ > after

real	1m12.130s
user	0m18.180s
sys	0m19.952s

Running the after case a second time is significantly faster:

behdad:src 130$ time fc-scan ~/fonts/ > after

real	0m24.825s
user	0m12.408s
sys	0m11.356s

Next I'm going to try to not even read loca...
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