[joystickcls] GetLastChange button diffs

Ensure that when generating changes between the last joystick state and
the current one that we actually compare button states.

This causes another bug with the joystick axis frequently becoming
selected, which will be fixed in a further commit

Joystick/JoystickCls.cs

@@ -219,8 +219,10 @@ namespace SCJMapper_V2
       if ( pov[3] >= 0 ) if ( pov[3] != ppov[3] ) m_lastItem = "hat4_" + HatDir( pov[0] );
 
       bool[] buttons = m_state.Buttons;
+      bool[] prevButtons = m_prevState.Buttons;
       for ( int bi=0; bi < buttons.Length; bi++ ) {
-        if ( buttons[bi] ) m_lastItem = "button" + ( bi + 1 ).ToString( );
+        if ( buttons[bi] && buttons[bi] != prevButtons[bi]) 
+            m_lastItem = "button" + ( bi + 1 ).ToString( );
       }
       return m_lastItem;
     }