validate mkv/CRC32 in mediatrace

[dericed]

Here are samples: Only Level 1 CRC32 [CRC32 Insanity!!!](http://archive.org/download/3asq.comBNXFANTASTECMidoHeRo/3asq.com-BNX-EP 01-[F A N T A S T E C-MidoHeRo].mkv_ma.xml.gz)

These use two very different approaches to CRC management in Matroska.

I suggest the expression of validity mirror what was done for FFV1. Possibly expressing the newly computed but invalid CRC would be helpful to mkv forensics.