Fix sound corruption with multiple sound files (#2368).

Fix sound corruption with multiple sound files (#2368).

last_push_end would be initialised to the position of the content,
meaning that content at (for example) 60s into the project would
be treated as the leader, and content much earlier would then be
considered finished by the code which checks ignore_streams_behind
(as its last push was more then 5s before the leader).

This caused audio corruption, shown up by the test here.