Remove last dependency on the GdkPixbuf API

Remove last dependency on the GdkPixbuf API

In test-pixmap.c instead of using the GdkPixbuf API to load the
redhand.png image we now use the cairo API to load the png into a xlib
surface which wraps our Pixmap.

This test was the last thing that depended on the gdk API and since
it's more concise to use Cairo here which is a hard dependency for
Clutter this change means we avoid depending on GdkPixbuf directly.