mono: Always bias initial edge advancement

Even for a vertical edge, we still need to bias the error term otherwise
later we will find the error term is too larger and advance a pixel on
every row.

Signed-off-by: Zhigang Gong <zhigang.gong@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/cairo-mono-scan-converter.c

@@ -200,8 +200,8 @@ polygon_add_edge (struct polygon *polygon,
 	e->x = floored_muldivrem (ytop * CAIRO_FIXED_ONE + CAIRO_FIXED_FRAC_MASK/2 - edge->line.p1.y,
 				  dx, dy);
 	e->x.quo += edge->line.p1.x;
-	e->x.rem -= dy;
     }
+    e->x.rem -= dy;
 
     _polygon_insert_edge_into_its_y_bucket (polygon, e, ytop);
 }