canvas: Allow invalidating the content along with the size

Currently, clutter_canvas_set_size() causes invalidation of the canvas
contents only if the newly set size is different. There are cases when
we want to invalidate the content regardless of the size set, but we
cannot do that right now without possibly causing two invalidations,
for instance:

  clutter_canvas_set_size (canvas, new_width, new_height);
  clutter_content_invalidate (canvas);

will cause two invalidations if the newly set size is different than
the existing one. One way to work around it is to check the current
size of the canvas and either call set_size() or invalidate() depending
on whether the size differs or not, respectively:

  g_object_get (canvas, "width", &width, "height", &height, NULL);
  if (width != new_width || height != new_height)
    clutter_canvas_set_size (canvas, new_width, new_height);
  else
    clutter_content_invalidate (canvas);

this, howevere, implies knowledge of the internals of ClutterCanvas,
and of its optimizations — and encodes a certain behaviour in third
party code, which makes changes further down the line harder.

We could remove the optimization, and just issue an invalidation
regardless of the surface size, but it's not something I'd be happy to
do. Instead, we can add a new function specifically for ClutterCanvas
that causes a forced invalidation regardless of the size change. If we
ever decide to remove the optimization further down the road, we can
simply deprecate the function, and make it an alias of invalidate()
or set_size().

clutter/clutter-canvas.h

@@ -89,11 +89,16 @@ CLUTTER_AVAILABLE_IN_1_10
 GType clutter_canvas_get_type (void) G_GNUC_CONST;
 
 CLUTTER_AVAILABLE_IN_1_10
-ClutterContent *        clutter_canvas_new              (void);
+ClutterContent *        clutter_canvas_new                      (void);
 CLUTTER_AVAILABLE_IN_1_10
-void                    clutter_canvas_set_size         (ClutterCanvas *canvas,
-                                                         int            width,
-                                                         int            height);
+void                    clutter_canvas_set_size                 (ClutterCanvas *canvas,
+                                                                 int            width,
+                                                                 int            height);
+
+CLUTTER_AVAILABLE_IN_1_16
+void                    clutter_canvas_invalidate_with_size     (ClutterCanvas *canvas,
+                                                                 int            width,
+                                                                 int            height);
 
 G_END_DECLS