Make sure the proper metadata are used when stap_a RTP packet sets its marker bit

[varsill]

This PR:

• fixes a bug of not setting the marker bit of STAP-A RTP packets if the final NALu of a STAP packet is the last NALu of an access unit

The bugfix updates the metadata in the stap_acc each time a new buffer is processed, so that to make sure that when the stap_acc gets flushed, its the metadata of the final buffer (that might have end_access_unit: true set) that is used in the set_marker function.