Never assume the API of libav* to be stable unless at least 1 week has passed since
the last major version increase.
The last version increases were:
libavcodec:  ?
libavdevice: ?
libavfilter: ?
libavformat: ?
libpostproc: ?
libswscale:  ?
libavutil:   2009-03-08


API changes, most recent first:

2009-06-01 - r19025 - lavc 52.30.0 - av_lockmgr_register()
  av_lockmgr_register() can be used to register a callback function
  that lavc (and in the future, libraries that depend on lavc) can use
  to implement mutexes. The application should provide a callback function
  the implements the AV_LOCK_* operations described in avcodec.h.
  When the lock manager is registered FFmpeg is guaranteed to behave
  correct also in a multi-threaded application.

2009-04-30 - r18719 - lavc 52.28.0 - av_free_packet
  av_free_packet() is no longer an inline function. It is now exported.

2009-04-11 - r18431 - lavc 52.25.0 - deprecate av_destruct_packet_nofree
  Please use NULL instead. This has been supported since r16506
  (lavf > 52.23.1, lavc > 52.10.0).

2009-04-07 - r18351 - lavc 52.23.0 - avcodec_decode_video/audio/subtitle
  The old decoding functions are deprecated, all new code should use the
  new functions avcodec_decode_video2(), avcodec_decode_audio3() and
  avcodec_decode_subtitle2(). These new functions take an AVPacket *pkt
  argument instead of a const uint8_t *buf / int buf_size pair.

2009-04-03 - r18321 - lavu 50.3.0 - av_fifo_space
  Introduce the av_fifo_space() function.

2009-04-02 - r18317 - lavc 52.23.0 - AVPacket
  Move AVPacket declaration from libavformat/avformat.h to
  libavcodec/avcodec.h.

2009-03-22 - r18163 - lavu 50.2.0 - RGB32 pixel formats
  Make the pixel formats: PIX_FMT_ARGB, PIX_FMT_RGBA, PIX_FMT_ABGR,
  PIX_FMT_BGRA, which were defined as macros, defined as enum
  PixelFormat values, and viceversa make PIX_FMT_RGB32,
  PIX_FMT_RGB32_1, PIX_FMT_BGR32, PIX_FMT_BGR32_1 defined as macros.
  Also make avcodec_get_pix_fmt() recognize the "rgb32" and "bgr32"
  aliases.
  Re-sort the enum PixelFormat list accordingly.
  This change breaks API/ABI backward compatibility.

2009-03-22 - r18133 - lavu 50.1.0 - PIX_FMT_RGB5X5 endian variants
  Add the enum PixelFormat values:
  PIX_FMT_RGB565BE, PIX_FMT_RGB565LE, PIX_FMT_RGB555BE, PIX_FMT_RGB555LE,
  PIX_FMT_BGR565BE, PIX_FMT_BGR565LE, PIX_FMT_BGR555BE, PIX_FMT_BGR555LE.

2009-03-21 - r18116 - lavu 50.0.0  - av_random*
  The Mersenne Twister PRNG implemented through the av_random* functions
  was removed. Use the lagged Fibonacci PRNG through the av_lfg* functions
  instead.

2009-03-08 - r17869 - lavu 50.0.0  - AVFifoBuffer
  av_fifo_init, av_fifo_read, av_fifo_write and av_fifo_realloc were dropped
  and replaced by av_fifo_alloc, av_fifo_generic_read, av_fifo_generic_write
  and av_fifo_realloc2.
  In addition, the order of the function arguments of av_fifo_generic_read
  were changed to match av_fifo_generic_write.
  The AVFifoBuffer/struct AVFifoBuffer may only be used in an opaque way by
  applications, they may not use sizeof() or directly access members.

2009-03-01 - r17682 - lavf 52.31.0 - Generic metadata API
  Introduce a new metadata API (see av_metadata_get() and friends).
  The old API is now deprecated and should not be used anymore. This especially
  includes the following structure fields:
    - AVFormatContext.title
    - AVFormatContext.author
    - AVFormatContext.copyright
    - AVFormatContext.comment
    - AVFormatContext.album
    - AVFormatContext.year
    - AVFormatContext.track
    - AVFormatContext.genre
    - AVStream.language
    - AVStream.filename
    - AVProgram.provider_name
    - AVProgram.name
    - AVChapter.title