This changes the following things for WOW detection:
* WOW files are converted from 669 and 6692WOW uses default finetune
and volume values in every sample. xmp now checks these to rule out
some false positives.
* WOW files always have a restart byte of 0, which is also checked now
for the same reason.
* 6692WOW occasionally creates a WOW file with an extra byte for
unknown reasons, so the length check now rounds down the filesize.
Some comments on these WOW quirks have also been added to the MOD loader.
Two new test files: one is a regular MOD file with extra data appended
that previously caused it to be detected as a WOW; the other is a WOW file
that 6692WOW added the extra byte to, causing it to be detected as a MOD.
