clutter: Enable negative offsets in delete surrounding text

The input method can assign a negative value to
clutter_input_method_delete_surrounding() to move the cursor to the left.
But Wayland protocol accepts positive values in delete_surrounding() and
GTK converts the values to the negative ones in
text_input_delete_surrounding_text_apply().

https://gitlab.gnome.org/GNOME/mutter/issues/539

clutter/clutter/clutter-input-focus.c

@@ -217,7 +217,7 @@ clutter_input_focus_commit (ClutterInputFocus *focus,
 
 void
 clutter_input_focus_delete_surrounding (ClutterInputFocus *focus,
-                                        guint              offset,
+                                        int                offset,
                                         guint              len)
 {
   g_return_if_fail (CLUTTER_IS_INPUT_FOCUS (focus));