From X-Plane SDK
XPLM_API int XPLMRegisterDrawCallback( XPLMDrawCallback_f inCallback, XPLMDrawingPhase inPhase, int inWantsBefore, void * inRefcon);This routine registers a low level drawing callback. Pass in the phase you want to be called for and whether you want to be called before or after. This routine returns 1 if the registration was successful, or 0 if the phase does not exist in this version of x-plane. You may register a callback multiple times for the same or different phases as long as the refcon is unique each time.