Round projs computed executing IP/ISECT instructions and improve IUP

Round projs computed executing IP/ISECT instructions and improve IUP

* src/sfnt.c (sfnt_multiply_divide_rounded): New function.
(sfnt_multiply_divide_signed): Always round values, as fonts
which rely on IP to move points in concert with prior motion and
subsequently round such points with MDAP are sensitive to minor
deviations in the behavior of the former instruction.
(load_unscaled): New macro.
(IUP_SINGLE_PAIR, sfnt_interpret_iup_1): Compute ratio w/
unscaled points if possible.