Final set of changes for a working ffmpeg.

Final set of changes for a working ffmpeg.

	There are a couple of edges to polish but "works for me" now.

	* src/media/media-ffmpeg.c (media_ffmpeg_read): Use
	avcodec_decode_audio3 if available. More work is needed to use
	avcodec_decod_audio4 because it uses frames which would need to
	then be copied to the output buffer.
	(my_get_buffer): Use avcodec_default_get_buffer2.
	(my_release_buffer): No longer needed.
	(stream_component_open): Properly set the thread count and use
	avcodec_open2. Set get_buffer2 to my_get_buffer since we
	are using the new interface.
	(media_ffmpeg_available_formats): Use av_iformat_next.
	(stream_open): Defer setting codec options to a later better
	understanding of ffmpeg (keep #if 0 the old).
	(new_media_ffmpeg_open): Use only the most recent AVIOContext access
	method to set eof_reached. If we get issues reported we'll
	revisit.
	(new_media_ffmpeg_read): Use avio_eof and the most error field of
	AVIOContext instead of furl_feof.  Do not unset the interrupt
	callback for now.  We may need to revisit the reset of the
	callback later.

	* m4/sxe-mm.m4 (SXE_CHECK_FFMPEG_LIBS): Properly check for the
	avcodec_decode_audio* and define corresponding
	HAVE_AVCODEC_AUDIO*.  Also check for av_iformat_next.
	(SXE_MM_CHECK_FFMPEG): Check at least one avcodec_decode_audio is
	available and av_iformat_next.

Signed-off-by: Nelson Ferreira <nelson.ferreira@ieee.org>