Fix bad phase seek when starting from preroll.

[ywwg]

We were interpreting all <0 playposition as "invalid", which is not correct.
fixes https://bugs.launchpad.net/mixxx/+bug/1930143

[ywwg]

This is a great example of where using magic numbers to mean invalid is bad!

[ywwg]

fwiw I don't think this is minor, this caused some trainwrecks in a set I played because the cue points were just barely before the start of the track.

[Holzhaus]

@ywwg can you take care of the merge to main after merging this PR?

[ywwg]

@ywwg can you take care of the merge to main after merging this PR?

yeah sure. I actually wrote it for that and then backported

[Holzhaus]

Works fine on my machine. Waiting for CI.

[ywwg]

what's the right way to merge to main? do a "git merge 2.3", or do a cherrypick of the commit?

[Holzhaus]

$ git checkout --track upstream/main
$ git pull upstream main  # this should be fast-forward or a noop
$ git pull upstream 2.3
$ make -j16 && ctest -j16
$ git push upstream main

[ywwg]

pushed to main