FcWeightFromOpenTypeDouble - Convert from OpenType weight values to fontconfig
  ones
#include <fontconfig/fontconfig.h>
double FcWeightFromOpenTypeDouble (doubleot_weight);
FcWeightFromOpenTypeDouble returns an double value to use with FC_WEIGHT,
  from an double in the 1..1000 range, resembling the numbers from OpenType
  specification's OS/2 usWeight numbers, which are also similar to CSS
  font-weight numbers. If input is negative, zero, or greater than 1000, returns
  -1. This function linearly doubleerpolates between various FC_WEIGHT_*
  constants. As such, the returned value does not necessarily match any of the
  predefined constants.