[autofit] Use unsigned accounting for hints. * src/autofit/afhints.h (AF_AxisHintsRec): Use unsigned types. * src/autofit/afhints.c (af_axis_hints_new_{segment,edge}, af_glyph_hints_get_num_segments, af_glyph_hints_get_segment_offset): Updated accordingly. * src/autofit/aflatin.c (af_cjk_hints_compute_edges): Ditto. * src/autofit/afcjk.c (af_cjk_hints_compute_edges): Ditto.