Add a new property so that clocksync can setup "ts-offset" value
based on the first buffer and pipeline's running time when the
first arrived. Newly update "ts-offset" in this case would be
a value that allows outputting the first buffer without clock waiting.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/702>
Since those are using the clock for sync, they need to also
provide a clock for good measure. The reason is that even if
downstream elements provide a clock, we don't want to have
that clock selected because it might not be running yet.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/509>
In reverse playback, buffers are played back from buffer.stop
(buffer.pts + buffer.duration) to buffer.pts running times which
mean that we need to use the buffer end running time as a buffer
timestsamp, not the buffer pts when using a single segment in reverse
playback.
This is now being tested in
`validate.test.identity.reverse_single_segment`
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/450>
The clocksync element is a generic element that can be
placed in a pipeline to synchronise passing buffers to the
clock at that point. This is similar to 'identity sync=true',
but because it isn't GstBaseTransform-based, it can process
GstBufferLists without breaking them into separate GstBuffers
