Add support for MKV cropping tag

The Matroska container features a tag for "soft" cropping of video streams 
(e.g. to compensate for black bars in the video stream without reencoding). 
VLC already supports this.

I implemented this by adjusting the frame size used for calculations in 
CMainFrame::MoveVideoWindow. A desirable side effect of this method
is that the user can "undo" the cropping by simply zooming out via Pan&Scan.

[This is my first time contributing to an open source project. I tried my best
to do everything right... Feedback/criticism is appreciated, but please be gentle.]