gapless: Fix seek race after about-to-finish

[adamcik]

Currently you can seek in the wrong track. This needs to be fixed so we change tracks upon seeking when it happens before stream changed. To test this I highly recommend setting the output queue time to 5-15s min/max making it easy to reproduce.

[adamcik]

Started adding a test and basic support for this. But currently running into the following issues:

• We end up emitting a a lot of events that shouldn't really trigger
• Just calling play() inside the seek() isn't really enough, we probably need to store a pending seek. And then trigger that at the right time.

[adamcik]

Fixed by #1346