[sfnt] Signedness fixes.

* src/sfnt/pngshim.c, src/sfnt/sfobjs.c, src/sfnt/ttcmap.c,
src/sfnt/ttkern.c, src/sfnt/ttload.c, src/sfnt/ttpost.c,
src/sfnt/ttsbit.c: Apply.
* src/sfnt/sfdriver.c: Apply.
(sfnt_get_ps_name): Simplify.
9 files changed