[raster] Speed up Bézier arches with extrema.

While it is recommended to have an explicit point at each curve
extrema, they might be missing or outline could be rotated. This
leads to excessive bisections in raster to find them.  This change
helps to decrease the number of bisections.  The scanline
intersections remain monotonous, of course.

* src/raster/ftraster.c (Conic_To, Cubic_To): Check that control
points cross the scanlines to bisect.

src/raster/ftraster.c

@@ -1342,7 +1342,7 @@
         ymax = y1;
       }
 
-      if ( y2 < ymin || y2 > ymax )
+      if ( y2 < FLOOR( ymin ) || y2 > CEILING( ymax ) )
       {
         /* this arc has no given direction, split it! */
         Split_Conic( arc );
@@ -1350,7 +1350,8 @@
       }
       else if ( y1 == y3 )
       {
-        /* this arc is flat, ignore it and pop it from the Bezier stack */
+        /* this arc is flat, advance position */
+        /* and pop it from the Bezier stack   */
         arc -= 2;
 
         ras.lastX = x3;
@@ -1488,7 +1489,7 @@
         ymax2 = y2;
       }
 
-      if ( ymin2 < ymin1 || ymax2 > ymax1 )
+      if ( ymin2 < FLOOR( ymin1 ) || ymax2 > CEILING( ymax1 ) )
       {
         /* this arc has no given direction, split it! */
         Split_Cubic( arc );
@@ -1496,7 +1497,8 @@
       }
       else if ( y1 == y4 )
       {
-        /* this arc is flat, ignore it and pop it from the Bezier stack */
+        /* this arc is flat, advance position */
+        /* and pop it from the Bezier stack   */
         arc -= 3;
 
         ras.lastX = x4;