Get rid of a signed-compare warning.

rect_i.x1 is signed but width is unsigned.  Casting width to signed is
consistent with the rest of the code.

Author: anntzer

src/_backend_agg.cpp

@@ -201,7 +201,7 @@ agg::rect_i RendererAgg::get_content_extents()
         }
     }
 
-    if (r.x1 == width && r.x2 == 0) {
+    if (r.x1 == (int)width && r.x2 == 0) {
       // The buffer is completely empty.
       r.x1 = r.y1 = r.x2 = r.y2 = 0;
     } else {