Really fix album replaygain calculation with gstreamer backend.

[sdrik]

Fixes #2845

[sampsyo]

Interesting; thanks!! Can you please describe a little bit about why this seems to be the right fix?

Also, please add a quick changelog entry if you have a moment.

[sdrik]

As I'm not a gstreamer expert this was almost trial and error, but the existing comment about "ensuring that the filesrc and decodebin elements received the paused state of the pipeline in a blocking manner" led me to the right direction.

What I understand is that even if the paused state is set in a blocking manner on the pipeline in _set_next_file, it is propagated asynchronously to its child elements (filesrc and decodebin), thus the sync_state_with_parent/get_state calls in _set_file.
But those synchronization calls were made after the unlink/set_state/set_property calls, leading to a race condition.
So my fix is simply to move the synchronization calls before any calls affecting filesrc and decodebin.
I also removed the added set_state(PLAYING) because it was obviously redundant with the one in _set_next_file.

[sampsyo]

Got it! That makes a lot of sense. Thank you for looking into it and finding this fix—trial-and-error is usually the way I sort things out with GStreamer too. ✨