Frame-based multithreading framework using pthreads

See doc/multithreading.txt for details on use in codecs.

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

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+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.
+
+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().