Ensure user is returned to the `RoomSubScreen` before gameplay is started

This covers the scenario where a user may be at the song select screen
while another user (the room host) starts the match. This was only made
possible with the new queue modes, so is quite a recent regression.

osu.Game/Screens/OnlinePlay/Match/RoomSubScreen.cs

@@ -319,6 +319,16 @@ public override bool OnExiting(IScreen next)
 
         protected void StartPlay()
         {
+            // User may be at song select or otherwise when the host starts gameplay.
+            // Ensure that they first return to this screen, else global bindables (beatmap etc.) may be in a bad state.
+            if (!this.IsCurrentScreen())
+            {
+                this.MakeCurrent();
+
+                Schedule(StartPlay);
+                return;
+            }
+
             sampleStart?.Play();
 
             // fallback is to allow this class to operate when there is no parent OnlineScreen (testing purposes).