Fix quantized play from the pre-roll in beatmap tracks

[daschuer]

This fixes https://bugs.launchpad.net/mixxx/+bug/1920084

It also fixes a bug that the user offset during sync was applied to the wrong deck.
And that quantize play syncs sometimes to the last beat rather than the next.

Tests are in place.

[Holzhaus]

Thanks. I've not tested this yet, but my impression was that root issue is that there ain't any beats before the track start. We should interpolate them from the beat distance between beat 1 and 2. This would also fix the issue that you can beatjump backwards from beat 1 into the Preroll. With beatgrids, this is possible.

[daschuer]

Exactly this is done here.

The beatloop issue remains. This need to be fixed in another place.

[uklotzde]

main or 2.3?

[daschuer]

Ups ... 2.3

[Holzhaus]

Exactly this is done here.

Doesn't work for me:

Left Deck: BeatMap (Preroll has no beat markers and does not support beatjumps)
Right Deck: BeatGrid (Preroll has beat markers and supports beatjumps)

I we don't have to fix it here, I'm fine merging this and then getting #2961 in a mergeable state for 2.4 ASAP.

[daschuer]

Ah, yes. I think this should be fixed by actually marking the beats in the preroll.

This here is a solution if you have regions with no beats. I just use the next possible beat in the future for matching them.

[daschuer]

Done.

[Holzhaus]

I think this doesn't work correctly (or I don't understand what this PR is supposed to fix). Here' s a video of the issue: https://www.youtube.com/watch?v=MKlzFQ_CpKU

[uklotzde]

@ywwg I am not familiar with the sync code.

No more concerns about the code in general.

[Holzhaus]

I think this doesn't work correctly (or I don't understand what this PR is supposed to fix). Here' s a video of the issue: https://www.youtube.com/watch?v=MKlzFQ_CpKU

@daschuer IIUC this PR fixes a different bug that let's the track jump to the first beat? If so, LGTM.

[daschuer]

Correct