add is_empty method to States

this is intended to allow a game loop to determine if the state
manager is actually doing anything or not, allowing the game loop to
exit out when it determines if there are no states and no more pending
states.

libretrogd/src/states/mod.rs

@@ -164,6 +164,11 @@ impl<ContextType> States<ContextType> {
         }
     }
 
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.states.is_empty() && self.pending_state.is_none() && self.command.is_none()
+    }
+
     fn can_push_or_pop(&self) -> bool {
         if let Some(state) = self.states.front() {
             if state.current_state != State::Active {