support data tracks at the start of the disc

This is mostly a PC game in track 1 and the sound for the game in the
other tracks.
Examples: Quake, AoE.
we can NOT just skip data tracks up front

MusicBrainz always expects a track list starting with the first track.
The cd spec also says the first track number should be one,
so we just take the track number in the toc.

No clue if there are releases out there with 1st track != 1 in the TOC.