[autofit] Improve recognition of flat vs. rounded segments. Lower the flatness threshold from upem/8 to upem/14, making the auto-hinter accept shorter elements. Synchronize flat/round stem selection algorithm with blue zone code. * src/autofit/aflatin.c (FLAT_THRESHOLD): New macro. (af_latin_metrics_init_blues): Use it. (af_latin_hints_compute_segments): Collect information on maximum and minimum coordinates of `on' points; use this to add a constraint for the flat/round decision similar to `af_latin_metrics_init_blues'.