fix overflow in qsort compare proc

git-svn-id: http://skia.googlecode.com/svn/trunk@335 2bbb7eff-a529-9590-31e7-b0007b416f81

src/core/SkScan_Path.cpp

@@ -383,7 +383,11 @@ extern "C" {
             valuea = edgea->fX;
             valueb = edgeb->fX;
         }
-        return valuea - valueb;
+
+        // this overflows if valuea >>> valueb or vice-versa
+        //     return valuea - valueb;
+        // do perform the slower but safe compares
+        return (valuea < valueb) ? -1 : (valuea > valueb);
     }
 }