Draw shadows for custom creature sprites

[Loobinex]

In 882e298f0bb89a890a58a28487628def791513cc the custom sprite functionality has been enabled for creatures. However, shadows for custom creatures are not drawn. This is explicitly disabled at the moment, because doing so would crash the game. '_DK_keepsprite' is a limiting factor here.

Shadows should be drawn on custom sprites the same way as on default sprites.