[raster] Rearrange dropout control.

This reduces the code duplication.

* src/raster/ftraster.c (Function_Sweep_Span): Change signature.
(Vertical_Sweep_Drop, Horizontal_Sweep_Drop): Focus on pixel setting
and move duplicated the dropout control logic to...
(Draw_Sweep): ... this function and refactor.
(Vertical_Sweep_Span, Horizontal_Sweep_Span): Minor.
1 file changed