Fix AGG crash when rendering extremely long horizontal lines.

When AGG rasterizes a horizontal line, it has code to check if the
computations will overflow its fixed-precision coordinate math. If an
hline is too wide, it will subdivide the line into two pieces until the
overflow is avoided.

Except a `return` statement was missing, so overflow-y code ran as well
as the subdivided code! Leading to negative coordinates and a crash.
This only happens for lines that span more than something like 32768
pixels in the X direction, so it's not something that happens in regular
usage.

Author: pkgw

extern/agg24-svn/include/agg_rasterizer_cells_aa.h

@@ -329,6 +329,7 @@ namespace agg
             int cy = (y1 + y2) >> 1;
             line(x1, y1, cx, cy);
             line(cx, cy, x2, y2);
+            return;
         }
 
         int dy = y2 - y1;