adaptivedemux: Handle prepared streams on seeks

adaptivedemux: Handle prepared streams on seeks

This is a race that was exposed by the {hls|dash}.scrub_forward_seeking
validate test.

The "race" is that a subclass might want to change format, causing
a new stream to be created (but not exposed/switched yet) and put on the
prepared_streams list. That stream will have values (including pending
segment) from the pre-seek state.

Before the stream is exposed/switched, a new seek comes in and the stream
values get updated ... but the ones that will be changed don't get updated
causing them to push out wrong segments once they are exposed.

https://bugzilla.gnome.org/show_bug.cgi?id=773159