resource-file constructors (using code formerly in Pixbuf.custom)
* gdk/Pixbuf.custom: Redo the stream and resource ctors in terms
of the PixbufLoader ones.
* gdk/PixbufAnimation.custom: Add stream and resource ctors
* gtk/Image.custom: Add stream and resource ctors
svn path=/trunk/gtk-sharp/; revision=42463