[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.