cvbsub: reorder the YUV order

The documentation doesn't mention the order is YVU.

Maybe at some point there was a bug in vcdimager and it got fixed according to the specs.

The VLC documentation says the CVD format is "closely related to SVCD". In SVCD the palette is read as YUVT. So it's a bit different than CVD which as YUV and Transparency separated.

Extracted from !3960