Use the timestamps directly as the ringbuffer index. We can save some
conversions to bytes and there is a direct mapping to RTP timestamp,
clock position and ringbuffer index.
Simplify the source a little. Remove the buffering state, we always
start with read and write pointers separted by the target buffering.
Always write to index of the clock, not just for the first packet.
This ensure the timestamp on the packet always matches the time it was
processed in the graph.
Always use the timestamp of the graph clock for RTP packets. Add an
option to apply a random or fixed offset.
Add a ts-refclk option on the sender to specify a reference clock to
use for timestamping. This will activate the direct timestamp mode and
signal this in the SDP.
Parse ts-refclk and ts-offset from the SDP. Make it possible to match
them in rules. Add option to activate the direct-timestamp, where the
rate matching is disabled and the timestamps are used directly for
writing and reading from the ringbuffer using the graph clock.
This makes it possible to set a PTP clock on sink and source and avoid
rate matching by using a shared clock.
