canvas: Add scale-factor property

We need to provide an escape hatch to ClutterCanvas so that it's
possible to override the window-scaling-factor ClutterSetting. This is
going to be useful in the future in case the user has better knowledge
of the window scaling factor that is going to be used with a specific
set of ClutterCanvas contents (e.g. on different outputs or stages).

https://bugzilla.gnome.org/show_bug.cgi?id=705915

clutter/clutter-canvas.h

@@ -95,6 +95,11 @@ gboolean                clutter_canvas_set_size                 (ClutterCanvas *
                                                                  int            width,
                                                                  int            height);
 
+CLUTTER_AVAILABLE_IN_1_18
+void                    clutter_canvas_set_scale_factor         (ClutterCanvas *canvas,
+                                                                 int            scale);
+CLUTTER_AVAILABLE_IN_1_18
+int                     clutter_canvas_get_scale_factor         (ClutterCanvas *canvas);
 
 G_END_DECLS