remove unused SDL2/SDL.h and physfs.h from twn_util.h, explicit usage of SDL in apps, move to Scancode enum in apps

include/twn_util.h

@@ -4,9 +4,6 @@
 #include "twn_vec.h"
 #include "twn_engine_api.h"
 
-#include <SDL2/SDL.h>
-#include <physfs.h>
-
 #include <stdint.h>
 #include <stdbool.h>