[autofit] Use `FT_LONG_MIN` and `FT_LONG_MAX` for `FT_Pos` variables. * src/autofit/aflatin.c (af_latin_metrics_init_blues, af_compute_vertical_extrema, af_find_highest_contour, af_find_second_highest_contour, af_find_lowest_contour, af_find_second_lowest_contour, af_check_contour_horizontal_overlap): Updated.