Port SRB2Kart join on intermission fix

All gamestates besides GS_LEVEL are unsupported by the save game functions. This commit forces players joining during these gamestates into GS_WAITINGPLAYERS, which is a basic gamestate that just maintains the connection until we can enter the start of a new one. Also provides an extremely simple drawer for GS_WAITINGPLAYERS so the joining player knows what's going on.