treeview: fix an off-by-one error

gtk_tree_view_get_path_at_pos() mistakenly identifies the first
pixel of all but the first column in a tree view as belonging to
the previous column.

https://bugzilla.gnome.org/show_bug.cgi?id=708148

gtk/gtktreeview.c

@@ -13564,7 +13564,7 @@ gtk_tree_view_get_path_at_pos (GtkTreeView        *tree_view,
 	  last_column = tmp_column;
           width = gtk_tree_view_column_get_width (tmp_column);
-	  if (remaining_x <= width)
+	  if (remaining_x < width)
 	    {
               found = TRUE;