fmp4: Return a running time in get_next_time()

We were currently returning a value based on the next chunk PTS, but the expectation in GstAggregator is that we return a running time. This resulted in spurious wakeups and warnings like:

0:00:01.501685123 1552995 0x55899715c1e0 WARN fmp4mux mux/fmp4/src/fmp4mux/imp.rs:1818:gstfmp4::fmp4mux::imp::FMP4Mux::drain_buffers:fmp4mux0:sink_1 Don't have a complete GOP for the first stream on timeout in a live pipeline

Depends on gstreamer-rs!1236 (merged)