rtpbasepayload: inconsistent units for running-time in stats

Submitted by Michael Tretter @m.tretter

Link to original bug (#793094)

Description

The rtpbasepayloader reports the running time of the last processed buffer in the stats property. The unit of running time can be either ns or rtptime ticks depending on the setting of perfect-rtptime.

The gst-rtsp-server uses the running time to calculate the clock base for the RTP-Info header and assumes that running time is in ns and scales it to rtptime ticks afterwards. If the streams do not start at the same time, this can lead to an erroneous clock-base in the RTP-info and out of sync streams on the receiver.

I would expect that the running time in the stats is always in ns, but I am not sure about the actually intended design. Even the test-case for the rptbasepayloader rtp_base_payload_property_stats_test() switches the units of running time depending on the setting of perfect-rtptime.