Let the Renderer decide when to exit fullscreen.

This ensures the transition from/to exclusive mode happens while the RenderFrame is fullscreen.

This prevents fullscreen loops and relieves us of having to restore the window size after we exit fullscreen.

Source/Core/Core/Host.h

@@ -31,6 +31,7 @@ void Host_Message(int Id);
 void Host_NotifyMapLoaded();
 void Host_RefreshDSPDebuggerWindow();
 void Host_RequestRenderWindowSize(int width, int height);
+void Host_RequestFullscreen(bool fullscreen);
 void Host_SetStartupDebuggingParameters();
 void Host_SetWiiMoteConnectionState(int _State);
 void Host_ShowJitResults(unsigned int address);