matroskaenc: Allow VP9 and Opus in webm

Bug-Id: 695

Signed-off-by: Anton Khirnov <anton@khirnov.net>
lavu: add all color-related enums to AVFrame

Signed-off-by: Anton Khirnov <anton@khirnov.net>
libavcodec: set AVFrame colorspace fields on decoding

Signed-off-by: Anton Khirnov <anton@khirnov.net>
lavc: add an option to enable side data-only packets during encoding

Some encoders (e.g. flac) need to send side data when there is no more
data to be output. This enables them to output a packet with no data in
it, only side data.
flacenc: send final extradata in packet side data
flac muxer: accept only STREAMINFO extradata

The other format (full flac header blocks) should not be exported by any
demuxers anymore.

This allows to drop an avpriv_ function and also simplify the following
commits.
flac muxer: support reading updated extradata from side data
avconv: do not use the stream codec context for decoding
avconv: do not use the stream codec context for encoding