lavc: introduce a new decoding/encoding API with decoupled input/output

Until now, the decoding API was restricted to outputting 0 or 1 frames
per input packet. It also enforces a somewhat rigid dataflow in general.

This new API seeks to relax these restrictions by decoupling input and
output. Instead of doing a single call on each decode step, which may
consume the packet and may produce output, the new API requires the user
to send input first, and then ask for output.

For now, there are no codecs supporting this API. The API can work with
codecs using the old API, and most code added here is to make them
interoperate. The reverse is not possible, although for audio it might.

From Libav commit 05f66706.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

doc/APIchanges

@@ -15,6 +15,11 @@ libavutil:     2015-08-28
 
 API changes, most recent first:
 
+2016-xx-xx - xxxxxxx - lavc 57.37.100 - avcodec.h
+  Add a new audio/video encoding and decoding API with decoupled input
+  and output -- avcodec_send_packet(), avcodec_receive_frame(),
+  avcodec_send_frame() and avcodec_receive_packet().
+
 2016-xx-xx - xxxxxxx - lavc 57.15.0 - avcodec.h
   Add a new bitstream filtering API working with AVPackets.
   Deprecate the old bistream filtering API.

libavcodec/internal.h

@@ -160,6 +160,19 @@ typedef struct AVCodecInternal {
      * hwaccel-specific private data
      */
     void *hwaccel_priv_data;
+
+    /**
+     * checks API usage: after codec draining, flush is required to resume operation
+     */
+    int draining;
+
+    /**
+     * buffers for using new encode/decode API through legacy API
+     */
+    AVPacket *buffer_pkt;
+    int buffer_pkt_valid; // encoding: packet without data can be valid
+    AVFrame *buffer_frame;
+    int draining_done;
 } AVCodecInternal;
 
 struct AVCodecDefault {

libavcodec/version.h

@@ -28,7 +28,7 @@
 #include "libavutil/version.h"
 
 #define LIBAVCODEC_VERSION_MAJOR  57
-#define LIBAVCODEC_VERSION_MINOR  36
+#define LIBAVCODEC_VERSION_MINOR  37
 #define LIBAVCODEC_VERSION_MICRO 100
 
 #define LIBAVCODEC_VERSION_INT  AV_VERSION_INT(LIBAVCODEC_VERSION_MAJOR, \

libavformat/avformat.h

@@ -161,8 +161,8 @@
  * av_read_frame() on it. Each call, if successful, will return an AVPacket
  * containing encoded data for one AVStream, identified by
  * AVPacket.stream_index. This packet may be passed straight into the libavcodec
- * decoding functions avcodec_decode_video2(), avcodec_decode_audio4() or
- * avcodec_decode_subtitle2() if the caller wishes to decode the data.
+ * decoding functions avcodec_send_packet() or avcodec_decode_subtitle2() if the
+ * caller wishes to decode the data.
  *
  * AVPacket.pts, AVPacket.dts and AVPacket.duration timing information will be
  * set if known. They may also be unset (i.e. AV_NOPTS_VALUE for