opusparse: overwrites upstream timestamps

Submitted by Tim Müller @tpm

Link to original bug (#754669)

Description

Opusparse seems to just make up its own timestamps by starting from 0 and adding the frame durations. This probably means it does not take into account any upstream timestamps in case where we're not reading straight from a file but there's a demuxer or depayloader upstream.

Just came across this whilst investigating follow-ups to bug 752106, did not look in detail, just filing it here for now before I forget.