[cff] Make blend operator work with floats in private dicts.

The CFF2 blend operator takes N default values and corresponding
sets of deltas and pushes N values specific to a designspace
location.  CFF has a floating point numeric type and the FreeType
blending code was not converting those into its internal 16.16
Fixed type format.

Fixes #1243.

* src/cff/cffparse.c (do_fixed): Handle floating point numbers.
Also fix scaling overflow check for integer-to-fixed conversion.

* src/cff/cffload.c (cff_blend_doBlend): Updated.