[smooth] Fix and improve spline flattening. 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.