Freetype expects its IO to perform like mmap (i.e. seeking to be free),
but the non-unix implementation uses fseek and fread. This
implementation simply reads the entire file into a buffer, as fonts
generally aren't very big.
Cuts the time to recreate the fontconfig cache on my machine by about
60%.
This eliminates the need for a msys environment to build any part of
Aegisub. Also happens to make building FFmpeg much faster (~3 minutes
vs. ~10 minutes for me).