[autofit] Start implementing hinting direction (up/down, down/up). Right now, it does nothing. * src/autofit/afscript.h: Add another parameter to `SCRIPT', specifying hinting direction. * src/autofit/afglobal.c, scr/autofit/afglobal.h, src/autofit/afpic.c, src/autofit/afranges.h, src/autofit/afshaper.c, src/autofit/aftypes.h: Extend `SCRIPT' definitions.