drawing bitmap with transparency

[jody <jody xha gmail com>]

Hi

I would like to draw an image with alpha channel into a Pixbuf., but
as far as i can see a pixbuf only has Gdk::COLORSPACE_RGB.
(in the end i want to be able to move an object around in front of a
background image in a Gtk::Image)
Is there some nice way to do this gtkmm 3?

Thank You
  Jody