FFmpeg multithreading methods

 ==============================================
 

 FFmpeg provides two methods for multithreading codecs.

 
 Slice threading decodes multiple parts of a frame at the same time, using
 AVCodecContext execute() and execute2().
 
 Frame threading decodes multiple frames at the same time.
 It accepts N future frames and delays decoded pictures by N-1 frames.
 The later frames are decoded in separate threads while the user is
 displaying the current one.
 
 Restrictions on clients
 ==============================================
 
 Slice threading -
 * The client's draw_horiz_band() must be thread-safe according to the comment
   in avcodec.h.
 
 Frame threading -
 * Restrictions with slice threading also apply.
 * For best performance, the client should set thread_safe_callbacks if it
   provides a thread-safe get_buffer() callback.
 * There is one frame of delay added for every thread beyond the first one.
   Clients must be able to handle this; the pkt_dts and pkt_pts fields in
   AVFrame will work as usual.
 
 Restrictions on codec implementations
 ==============================================
 
 Slice threading -
  None except that there must be something worth executing in parallel.
 
 Frame threading -
 * Codecs can only accept entire pictures per packet.
 * Codecs similar to ffv1, whose streams don't reset across frames,
   will not work because their bitstreams cannot be decoded in parallel.
 
 * The contents of buffers must not be read before ff_thread_await_progress()
   has been called on them. reget_buffer() and buffer age optimizations no longer work.
 * The contents of buffers must not be written to after ff_thread_report_progress()
   has been called on them. This includes draw_edges().
 
 Porting codecs to frame threading
 ==============================================
 
 Find all context variables that are needed by the next frame. Move all
 code changing them, as well as code calling get_buffer(), up to before
 the decode process starts. Call ff_thread_finish_setup() afterwards. If
 some code can't be moved, have update_thread_context() run it in the next
 thread.
 
 If the codec allocates writable tables in its init(), add an init_thread_copy()
 which re-allocates them for other threads.
 
 Add CODEC_CAP_FRAME_THREADS to the codec capabilities. There will be very little
 speed gain at this point but it should work.
 

 If there are inter-frame dependencies, so the codec calls
 ff_thread_report/await_progress(), set AVCodecInternal.allocate_progress. The
 frames must then be freed with ff_thread_release_buffer().
 Otherwise leave it at zero and decode directly into the user-supplied frames.
 

 Call ff_thread_report_progress() after some part of the current picture has decoded.
 A good place to put this is where draw_horiz_band() is called - add this if it isn't
 called anywhere, as it's useful too and the implementation is trivial when you're
 doing this. Note that draw_edges() needs to be called before reporting progress.
 
 Before accessing a reference frame or its MVs, call ff_thread_await_progress().