Fixup for selection-data where format=32. X specification is odd.

Fixup for selection-data where format=32.  X specification is odd.
Added basic, ugly convenience function for cairo->pixbuf conversion,
so cursors can be drawn on a cairo image surface.  gtk-drag-dest-set
now passes (null-pointer) if targets is nil; this invokes a special
case in gtk where all dragging can be handled by the client.

Conflicts:

	cairo/cairo.lisp
	gdk/gdk.pixbufs.lisp