lavc: Add option to encode MPEG-2 AAC with libfdk-aac

Signed-off-by: Anton Khirnov <anton@khirnov.net>
matroskaenc: restore compatibility with non referenced AVPacket

Signed-off-by: Anton Khirnov <anton@khirnov.net>
lavf: don't abort if both encoder and muxer aspect ratios are not set

Signed-off-by: Anton Khirnov <anton@khirnov.net>