[autofit] More tilde stretch support. (1/3) Move some existing code that we soon need elsewhere into separate functions. * src/autofit/aflatin.c (af_move_contours_up, af_move_contours, down): New functions. (af_glyph_hints_apply_vertical_separation_adjustments): Use them. (af_touch_contour): New function. (af_latin_stretch_top_tilde, af_latin_stretch_bottom_tilde): Use it. (af_latin_remove_top_tilde_points_from_edges, af_latin_remove_bottom_tilde_points_from_edges): Renamed to... (af_remove_top_points_from_edges, af_remove_bottom_points_from_edges): ...this. Update callers.