matroska: list supported extensions
avcodec: add stream-level stereo3d side data
matroskadec: parse stereo mode on decoding

Convert the Matroska stereo format to the Stereo3D format, and add a
Stereo3D side data to the stream.

Bump the doctype version supported.

Bug-Id: 728 / https://bugs.debian.org/757185
matroskaenc: convert avstream stereo3d side data during encoding

Write the StereoMode Embl to bitstream.