Add draw priority

Rather than blindly taking one entity from the list when we have
multiple entities on the same tile, add a `drawPriority` method to the
Draw typeclass which allows individual entities to request to be drawn
on top - this avoids the "noodles floating over your head" bug we saw
before.

src/Xanthous/Entities/Character.hs

@@ -50,6 +50,7 @@ instance Draw Character where
     where
       rloc = Location (negate scrollOffset, negate scrollOffset)
       rreg = (2 * scrollOffset, 2 * scrollOffset)
+  drawPriority = const maxBound -- Character should always be on top, for now
 
 -- the character does not (yet) have a mind of its own
 instance Brain Character where step = brainVia Brainless