videorate: add start-time and start-time-selection properties
Currently, the videorate output buffer timestamps always start form 0. This can be a problem when synchronizing audio and video in custom sinks if the pipeline was run and videorate has been waiting for input buffers for a log time (eg srtsrc in listen mode).
Similar to GstAggregator, this NR adds new start-time
and start-time-selection
properties to avoid this and so allow users have more control in the output buffer timestamps.