Provide a fallback for embedding widgets into actors

Currently, we can only embed a GtkWidget into a ClutterActor on X11 by
using a Pixmap and the GLX texture_from_pixmap extension. There is no
reason why we shouldn't be able to allow the same to happen by using
Cairo surfaces — except the performance implications of making copies of
the contents of the Drawable used by GDK.

This should allow embedding widgets on platforms that do not have the
texture_from_pixmap extension.
actor: Update after the API change in Clutter

The unstable ClutterCanvas API was changed, so we need to update
GtkClutterActor to use it.
Bump to 1.15.1
Bump up the Clutter required version