Preliminary 4-player counter-op mode

Here's some things that will be needed off the top of my head:

- Handling the half-screen-width case for the pause menus
- Changing the player select menu to select the not-counter-op player instead, then extending that to 4 players
- Thorough testing of each level for any weird counter-op-specific features

src/include/constants.h

@@ -20,8 +20,9 @@
 #define MAX_CHRWAYPOINTS       6
 #define MAX_EXPLOSIONS         6
 #define MAX_EYESPYDARTS        8
+#define MAX_COOPCHRS           MAX_PLAYERS
 #define MAX_MPCHRS             (MAX_PLAYERS + MAX_BOTS)
-#define MAX_MPPLAYERCONFIGS    (MAX_PLAYERS + 2)
+#define MAX_MPPLAYERCONFIGS    (MAX_PLAYERS + MAX_COOPCHRS)
 #define MAX_OBJECTIVES         10
 #define MAX_LOCAL_PLAYERS      4
 #define MAX_PLAYERS            8