Support pixelcrop values in Matroska

It appears that VLC (version 3.0.4 on OSX was tested) ignores pixelcrop values. An examples can be downloaded here and I will try to attach. https://archive.org/details/10sec

This video file was created with ffmpeg v4.0.2 using:

$ ffmpeg -f lavfi -i testsrc=size=720x576 -t 10 -pix_fmt yuv420p 10sec.mkv

PixelCrop values were then added using mkvpropedit:

$ mkvpropedit --edit track:v1 --set pixel-crop-left=100 --set pixel-crop-right=234  10sec.mkv

which produces these values via mediainfo:

Width                                    : 386 pixels
Original width                           : 720 pixels
Height                                   : 576 pixels

but no crop is present in vlc 3.0.4 on oSX.

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