This fixes the flattening of cubic, S-shaped curves and speeds up
the handling of both the conic and cubic arcs.
See the discussions on the freetype-devel mailing list in late
August and September 2010 for details.
* src/smooth/ftgrays.c (FT_MAX_CURVE_DEVIATION): New macro.
(TWorker): Remove `conic_level' and `cubic_level' elements.
(gray_render_conic): Simplify algorithm.
(gray_render_cubic): New algorithm; details are given in the code
comments.
(gray_convert_glyph): Remove heuristics.