[autofit] Use more functions to reduce code. * src/autofit/aflatin.c (af_find_highest_contour, af_find_lowest_contour): Updated. (af_glyph_hints_apply_vertical_separation_adjustments): Use `af_compute_vertical_extrema`, `af_find_highest_contour`, and `af_find_lowest_contour`. (af_latin_hints_apply): Use `af_compute_vertical_extrema`,