Fix two cases of signed integer overflow.

This tweaks two cases where there is potential for signed integer overflow:
* in `rewind_scanlines`, we switch to unsigned ints
* in `not_equal`, we switch for operations which cannot overflow.

These cases were caught by matplotlib-using tests being run under
[ASan](https://github.com/google/sanitizers) with a check for signed integer
overflow.

Author: craigcitro

extern/agg24-svn/include/agg_rasterizer_scanline_aa_nogamma.h

@@ -60,7 +60,7 @@ namespace agg
 
         int not_equal(int ex, int ey, const cell_aa&) const
         {
-            return (ex - x) | (ey - y);
+            return ex != x || ey != y;
         }
     };
 

extern/agg24-svn/include/agg_scanline_storage_aa.h

@@ -720,10 +720,10 @@ namespace agg
             m_ptr = m_data;
             if(m_ptr < m_end)
             {
-                m_min_x = read_int32() + m_dx; 
-                m_min_y = read_int32() + m_dy;
-                m_max_x = read_int32() + m_dx;
-                m_max_y = read_int32() + m_dy;
+                m_min_x = read_int32u() + m_dx;
+                m_min_y = read_int32u() + m_dy;
+                m_max_x = read_int32u() + m_dx;
+                m_max_y = read_int32u() + m_dy;
             }
             return m_ptr < m_end;
         }