Add exit semaphore to GameMode to ensure a mode can't be exited multiple times.
Add exit semaphore to GameMode to ensure a mode can't be exited multiple times.


EVAST9919/osu-framework,ppy/osu-framework,ppy/osu-framework,paparony03/osu-framework,Nabile-Rahmani/osu-framework,smoogipooo/osu-framework,Tom94/osu-framework,peppy/osu-framework,ZLima12/osu-framework,default0/osu-framework,RedNesto/osu-framework,smoogipooo/osu-framework,peppy/osu-framework,EVAST9919/osu-framework,paparony03/osu-framework,DrabWeb/osu-framework,ZLima12/osu-framework,naoey/osu-framework,peppy/osu-framework,DrabWeb/osu-framework,RedNesto/osu-framework,NeoAdonis/osu-framework,EVAST9919/osu-framework,ppy/osu-framework,Tom94/osu-framework,DrabWeb/osu-framework,NeoAdonis/osu-framework,naoey/osu-framework,default0/osu-framework,EVAST9919/osu-framework,Nabile-Rahmani/osu-framework