added general connection overview in header; introduced defines MMAL_CAMERA_xxx_PORT as found in raspistill and raspivid to get an easier overview of the connections to the camera output ports
stop_all() considers now all missing components, ports, pools, connections and their order (disable, destroy) as found in raspistill; all global components, ports, pools, connections are pre-initialized to NULL --> tried to improve overall stability
NULL setting of h264encoder; on error() stop_all() gets called to free any open ressources