Adds directional blood trails

The blood trails now follow the path the body has taken and players can
very easily make out said path.
Heavily damaged corpses will have a much thicker blood trail than
corpses with some brute damage. At the moment, trails overwrite
eachother and a placeholder icon set is used for alien heavy blood
trails. 

Blood.dmi contain's Cheridan's splatter sprites for future use. 
Uses/based on Pewtershmitz's blood sprites.