Setting state to GST_STATE_NULL sometimes blocks, to fix this use the threadpool to set the state to NULL and wait with deleting the pipeline until the state is changed.
This fixes blocking the main thread when switching Spotify songs.
Ok, it does appear that it is that simple.
In principle this (even the non-update case) results in volume jumps,
so maybe we'll want gradual gain change...
Notably, i thought we'd always seek if the pipeline
was already operating on the same URL as the new one,
but apparently only for adjacent songs?
This is a bit of a gotcha, there should be a point (where we seek?)
where we'd be able to change said gain, but for now this is a simple[r]
stop-gap fix.
The idea is that Integrated Loudness is an integral part
of the song, much like knowing it's beginning / ending
in the file, and we must handle it the exact same way,
and pipe it through all the way.
At the same time, `EngineBase` knows Target Level (from settings),
and these two combined tell us the Gain needed to normalize the
Loudness of the particular Song (`EngineBase::Load()` does that).
So the actual backend only needs to handle the Volume.
We don't currently support changing Target Level on the fly.
We don't currently support changing Loudness-normalizing Gain on the fly.
This does not handle the case when the song is loaded from URL
and thus the EBU R 128 measures, that exist, are not nessesairly correct.
If "about-to-finish" was emitted before the preload time was reached, we never set the next uri, so gapless playback was broken.
Make sure to always set the next uri, and increase preload gap from 5 to 8 seconds.
