Fixed display width calculation for matroska stereo mode.

When you encoding stereoscopic video pair, in side-by-side mode(left_right or right_left), you need to set display width to width of one pane(half of video width).
It described in matroska spec: 

> The pixel count of the track (PixelWidth/PixelHeight) should be the raw amount of pixels (for example 3840x1080 for full HD side by side) and the DisplayWidth/Height in pixels should be the amount of pixels for one plane (1920x1080 for that full HD stream).

http://matroska.org/technical/specs/notes.html#3D

I encountered wrong display width when try to stereoscopic webm video, which you can play in FF4+ on windows, if you have stereo display and NVIDIA 3D Vision compatible video(yep so much requirements :confused:).

http://blog.webmproject.org/2011/05/introducing-3d-webm-support-with-nvidia.html