Agg: Fix overflow when splitting large lines

For very large Figures with a single Axes, the horizontal spines may be
long enough to need splitting into separate lines. For large enough
coordinates, the `x1+x2` calculation may overflow, flipping the sign
bit, causing the coordinates to become negative. This causes an infinite
loop as some internal condition is never met.

By dividing `x1`/`x2` by 2 first, we avoid the overflow, and can
calculate the split point correctly.

Author: QuLogic

extern/agg24-svn/include/agg_rasterizer_cells_aa.h

@@ -325,8 +325,10 @@ namespace agg
 
         if(dx >= dx_limit || dx <= -dx_limit)
         {
-            int cx = (x1 + x2) >> 1;
-            int cy = (y1 + y2) >> 1;
+            // These are overflow safe versions of (x1 + x2) >> 1; divide each by 2
+            // first, then add 1 if both were odd.
+            int cx = (x1 >> 1) + (x2 >> 1) + ((x1 & 1) & (x2 & 1));
+            int cy = (y1 >> 1) + (y2 >> 1) + ((y1 & 1) & (y2 & 1));
             line(x1, y1, cx, cy);
             line(cx, cy, x2, y2);
             return;