Additional GDK-pixbuf loaders

[Magnus Bergman <magnusbe algonet se>]

I wrote a GDK-pixbuf loader for ILBM pictures the other day. And I
wonder if it's considered useful enough to be included in GTK? In that
case I will make it more complete.

I will probably also write loaders for PI1 and NEO pictures too soon,
but I guess those formats are considered to be far too obsqure for most
people.