- use AV_NOPTS_VALUE instead of 0 when it's available
  - reworked the hurry-up mode to start with disabling the B frames first
  - reworked the hurry-up mode to remove my thread mess
  - check that we don't feed ffmpeg with twice the same PTS (crashes ffmpeg)
  - correctly flag the picture types

* modules/stream_out/transcode.c:
  - use AV_NOPTS_VALUE instead of 0 when it's available
  - copy block_t::i_flags to sout_buffer_t::i_flags (temporary)