splitmuxsink: Take released-but-not-yet-output bytes into account

splitmuxsink: Take released-but-not-yet-output bytes into account

When deciding whether it's time to switch to a new file, take into
account data that's been released for pushing, but hasn't yet
been pushed - because downstream is slow or the threads haven't been
scheduled.

Fixes a race in the unit test and probably in practice - sometimes
failing to switch when it should for an extra GOP or two.

Also fix a problem in splitmuxsrc where playback sometimes
stalls at startup if types are found too quickly.

https://bugzilla.gnome.org/show_bug.cgi?id=750747