Handle CRC32 and Void elements in SEEK elements

Right now they are skipped (and only in SEEK elements), maybe they should be read and their value matched against the CRC32 of the whole element. But mpv isn't doing that, and it adds code.

Right now, the tags are detected if they are in a MKS file, but not if they are in a MKV file. Moreover, it's not deterministic, sometimes they are detected in a MKV file.