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.