H.264 error resiliency

Tasks

• to prevent running out of the bitrate, H.264 frames must be as even as possible
• let decoder process even partially corrupted frames
  • test which decoders crash on scrambled data
  • zero buffer space where data are missing due to missing packets (maybe not needed but more safe)
• divide RTP packets according to H.264 NAL units (like it is already for RFC-compliant transmission)
• consider fetching the lavd not H.264 frames but NAL units/packets
• consider invalidating incomplete NAL units