Simplify usMaxContext calc some, taking from FDK
Simplify usMaxContext calc some, taking from FDK

This attempts to do the same calculations as those in the FDK's
hotconv, though testing has determined that it still calculates
different values at times.

jamesgk/ufo2fdk,jamesgk/ufo2ft,googlefonts/ufo2ft,moyogo/ufo2ft,googlei18n/ufo2ft