Implement turn based mode

Seems to work mechanically though I haven't extensively stress tested it yet. Force floors work the way you'd want them to though (you're given control whenever you can make an input and not otherwise).
There are some graphical bugs with rewinding, but there were some without turn based mode anyway...

index.html

@@ -120,6 +120,7 @@
                 <button class="control-restart" type="button">Restart</button>
                 <button class="control-undo" type="button">Undo</button>
                 <button class="control-rewind" type="button">Rewind (z)</button>
+                <input class="turn-based" type="checkbox">Turn-Based</input>
             </div>
             <div class="demo-controls">
                 <button class="demo-play" type="button">View replay</button>