This callback allows client applications to directly render the gray spans of the anti-aliased bitmap to any kind of surfaces.
This can be used to write anti-aliased outlines directly to a given background bitmap, and even perform translucency.
Note that the `count' field cannot be greater than a fixed value defined by the FT_MAX_GRAY_SPANS configuration macro in ftoption.h. By default, this value is set to 32, which means that if there are more than 32 spans on a given scanline, the callback will be called several times with the same `y' parameter in order to draw all callbacks.
Otherwise, the callback is only called once per scan-line, and only for those scanlines that do have `gray' pixels on them.