WIP: baseparse: Don't ignore repeated incoming PTS

If the input stream has PTS set, then previously baseparse would only take a new timestamp when it changes, but it's not clear why it does that.

When parsing video streams this behaviour makes it lose timestamps from incoming video frames if there are duplicates due to upstream timestamping problems.

I'm marking this as WIP to get some discussion - I'm not sure why baseparse had this behaviour in place, but it dates way back to when it was aacparse in 48b4912c