Enable the new font scaler

Enable the new font scaler

* src/sfnt.c (sfnt_curve_is_flat): Tighten threshold for flat
curves.
(sfnt_insert_raster_step): Rewrite loop for clarity.
(sfnt_poly_set_steps): New function; fill small spans with a
plain unexceptional loop rather than memset.
(sfnt_poly_steps): Call that function.
(sfnt_verbose, main): Adjust tests such that the scaler can be
selected at runtime.

* src/sfnt.h: Update prototypes.

* src/sfntfont.c (sfntfont_get_glyph_raster)
(syms_of_sfntfont) <sfnt_raster_glyphs_exactly>: New variable.