Register a callback to be fired whenever the app is put into the foreground.
GLVIEW_API int glview_register_foreground_callback(foreground_callback foreground_callback)
The callback to be fired on a foreground event. If this parameter is NULL then any previously set callback is disabled.
Calling glview_register_foreground_callback() with a NULL foreground callback will disable any previously set callback from being fired on a foreground event.