Fix a thinko in computing damage for segments

The code was not taking the endpoint of the first segment into account.
This was reported in bug 604747.
(cherry picked from commit aadb190b77)

gdk/gdkoffscreenwindow.c

@@ -514,7 +514,7 @@ gdk_offscreen_window_draw_segments (GdkDrawable  *drawable,
       min_x = max_x = segs[0].x1;
       min_y = max_y = segs[0].y1;
 
-	for (i = 1; i < nsegs; i++)
+	for (i = 0; i < nsegs; i++)
 	  {
 	    min_x = MIN (min_x, segs[i].x1);
 	    max_x = MAX (max_x, segs[i].x1);