Revert to signed span positioning.

In the context of FT_Outline_Render, the outline can be placed
arbitrarily and the resulting spans should be signed. This partially
reverts ef04e4eb2056 and fixes #1410.  However, for internal use,
when outlines are placed in the first quadrant, we still can interpret
positions as unsigned.

* include/freetype/ftimage.h (FT_Span): Use signed position again.
* src/smooth/ftgrays.c (gray_sweep_direct): Sync with FT_Span.
* src/smooth/ftsmooth.c (ft_smooth_lcd_spans, ft_smooth_lcdv_spans,
ft_smooth_overlap_spans): Prevent sign extension.