Remove mainmenu from Game

Since mainmenu is only ever used in Input.cpp, I might as well make it
clearer by moving it into a static global variable in Input.cpp. (The
same applies to fadetolab/fadetomenu, but I didn't think much about
those at the time... that'll be a refactor for later.)

desktop_version/src/main.cpp

@@ -505,7 +505,6 @@ int main(int argc, char *argv[])
     game.gamestate = PRELOADER;
 
     game.menustart = false;
-    game.mainmenu = 0;
 
     // Initialize title screen to cyan
     graphics.titlebg.colstate = 10;