video: initial Matroska 3D support

This inserts an automatic conversion filter if a Matroska file is marked
as 3D (StereoMode element). The basic idea is similar to video rotation
and colorspace handling: the 3D mode is added as a property to the video
params. Depending on this property, a video filter can be inserted.

As of this commit, extending mp_image_params is actually completely
unnecessary - but the idea is that it will make it easier to integrate
with VOs supporting stereo 3D mogrification. Although vo_opengl does
support some stereo rendering, it didn't support the mode my sample file
used, so I'll leave that part for later.

Not that most mappings from Matroska mode to vf_stereo3d mode are
probably wrong, and some are missing.

Assuming that Matroska modes, and vf_stereo3d in modes, and out modes
are all the same might be an oversimplification - we'll see.

See issue #1045.
csputils: adjust comment

Make it clear that the value is linked to the StereoMode element. You
can't change this without adjusting demux_mkv.c.
manpage: be more explicit about "estimated-..." properties

In particular, use the note markup. The issue about rounded timestamps
is mostly with respect to Matroska (which usually rounds them to
milliseconds), which somewhat adds to the reliability issue.