Eliminate stacks disconnected from red DVONN pieces

At this point, the game is actually playable for the first time.
There's still very little actual UI here, (no indication of who
the current player is, no buttons for passing, starting a new
game, etc.). But with two people who know how to right-click
and have out-of-band communication to say when its the
opponent's turn, it's possible to play now.