track/serato/markers: Fix parsing of position values

[Holzhaus]

The cue position values in the Serato Markers_ tag use the same strange
4-byte encoding of 24 bit values that track colors use.

Prerequesite for #2526.

[uklotzde]

They did this on purpose. Bastards ;)

[Holzhaus]

Yeah, I wonder if it's to prevent reverse engineering or if it's for some kind of "poor man's integrity check".

[Holzhaus]

Most of the changes are renaming variables/function, modifying comments and fixing the tests. The actual changes to the position parsing are small: https://github.com/mixxxdj/mixxx/pull/2671/files#diff-ac00585ee405309584d3c94b489ffe17R122-R150

[Be-ing]

I wouldn't assume it's on purpose. It could be accidental, but it happens to work with whatever code they're using.

[Be-ing]

CI succeeded 👍

[Be-ing]

Maybe the tests in Uwe's branch will work now with this fix?

[uklotzde]

I'll retry to parse and dump the tag obtained from a FLAC file.

[Holzhaus]

AFAIK the FLAC files only have Serato Markers V2, not the first version, so it should be unaffected.
https://github.com/Holzhaus/serato-tags/blob/master/docs/fileformats.md#flac

[uklotzde]

Still differences. The first bytes match, then some differences occur.

Applying the base64 encoding twice on some parts of their marker data is a strange decision by Serato.

Original:
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Parsed and dumped:
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