Don't generate SDL events on Atlas shutdown

WX may still generate mouse events after calling CVideoMode::Shutdown(),
so don't create SDL events triggering assertions.

Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>

source/ps/VideoMode.cpp

@@ -963,6 +963,11 @@ SDL_Window* CVideoMode::GetWindow()
 	return m_Window;
 }
 
+bool CVideoMode::IsInitialized() const
+{
+	return m_IsInitialised;
+}
+
 void CVideoMode::SetWindowIcon()
 {
 	// The window icon should be kept outside of art/textures/, or else it will be converted