input: raise maximum key down limit to 16

The current limit of 4 is stupidly low, and won't be future proof
in case proper multi-touch support is added.

input/input.c

@@ -56,7 +56,7 @@
 #define input_lock(ictx)    mp_mutex_lock(&ictx->mutex)
 #define input_unlock(ictx)  mp_mutex_unlock(&ictx->mutex)
 
-#define MP_MAX_KEY_DOWN 4
+#define MP_MAX_KEY_DOWN 16
 
 struct cmd_bind {
     int keys[MP_MAX_KEY_DOWN];