Set up TextDecoder position based on the time that the next thing will

Set up TextDecoder position based on the time that the next thing will
be emitted, instead of the time that the last thing was (#2268).

This is to avoid problems with the example shown in the test, where
just because a subtitle in source A comes before a subtitle in source B,
source A is pass()ed next and may then emit a subtitle which should
be after the next one in B.