[smooth] Introduce direct oversampling for overlaps.

This implements oversampling to metigate artifacts in pixels partially
covered by overlapping contours.  It turns out that the 4x4
oversampling is sufficient but, at least, quadruples the rendering
time.  The outline has to set FT_OUTLINE_OVERLAP to use this method.

* include/freetype/ftimage.h (FT_OUTLINE_OVERLAP): New flag.
* src/smooth/ftsmooth.c (ft_smooth_render): Check it to...
(ft_smooth_raster_overlap): ... inflate outline and set up direct
rendering for oversampling with...
(ft_smooth_overlap_spans): ... new span function that integrates them.
3 files changed