ClutterImage is a ClutterContent implementation that
has an internally managed CoglTexture. This texture
is recreated when new image data is set.

ClutterContent implementations may have control over
the allocation of the widgets they're attached to,
through CLUTTER_REQUEST_CONTENT_SIZE. On those cases,
if the new image data differs in size from the previous
data, it is important to notify those actors about the
size change. However, currently ClutterImage does not
notify them.

With the introduction of clutter_content_invalidate_size(),
it is possible to report the size changes to attached
actors.

Adapt ClutterImage to invalidate_size() when image data
has different sizes.

GNOME/mutter!405