[spans] Round rasterization grid pixel areas instead of truncating.

When converting a grid pixel area into the range [0,255] the
GRID_AREA_TO_ALPHA() macro would truncate extra bits off the result
rather than rounding. This could cause seams between abutting
collinear edges of separately rendered polygons even when the
coordinates of the abutting edges were the same.

Reported by Soeren Sandmann on the cairo mailing list:

http://lists.cairographics.org/archives/cairo/2009-May/017043.html

src/cairo-tor-scan-converter.c

@@ -295,9 +295,9 @@ typedef int grid_area_t;
 #elif GRID_XY == 15
 #  define  GRID_AREA_TO_ALPHA(c)  (((c) << 4) + (c))
 #elif GRID_XY == 2*256*15
-#  define  GRID_AREA_TO_ALPHA(c)  (((c) + ((c)<<4)) >> 9)
+#  define  GRID_AREA_TO_ALPHA(c)  (((c) + ((c)<<4) + 256) >> 9)
 #else
-#  define  GRID_AREA_TO_ALPHA(c)  ((c)*255 / GRID_XY) /* tweak me for rounding */
+#  define  GRID_AREA_TO_ALPHA(c)  (((c)*255 + GRID_XY/2) / GRID_XY)
 #endif
 
 #define UNROLL3(x) x x x