Somethingonplayer

If a slimer is currently attached to the player, this holds the slimer's sprite ID. Otherwise, this is -1.

To kill the slimer from code, we can't just apply htextra as we would expect, the correct way to kill a facehugging slimer is this:

   ifn player[].somethingonplayer -1
   {
       seta[player[].somethingonplayer].htextra 1
       seta[player[].somethingonplayer].htg_t 0 0
       setp[].somethingonplayer -1
   }