Exotherm - Avoiding stalemate

Previously Exotherm's evaluation function knew nothing about stalemate. I once tried to add that consideration by checking the size of the list of legal moves, but generating that list is very expensive. I've never seen Exotherm stalemate its opponent, so that definitely wasn't worth it. But just to be safe, today I added stalemate detection in a different spot. Now it will retroactively score a position as zero (a draw) if it finds no legal moves when trying to deepen the tree.