By using a new signal, ::create-surface (width, height), it should be
possible for third party code and sub-classes to override the default
surface creation code in CairoSurface.
This commit takes a bit of the patch from:
http://bugzilla.clutter-project.org/show_bug.cgi?id=1878
which cleans up CairoTexture; the idea, mutuated from that bug, is that
the CairoTexture actor checks whether the surface it has it's an image
one, and in that case it uses a Cogl texture as the backing store. In
case the surface is not an image one we assume that the surface itself
has some way of updating the GL state and flush the surface.
