mpegtsmux: Start last_ts with GST_CLOCK_TIME_NONE
And use the output segment position for the outgoing timestamp while it
is. This is needed to delay the calculation of output_ts_offset
until
we actually have a usable timestamp, as tsmux will output a few initial
packets while last_ts
is still unset.
Without this, the calculation would use the initial 0
value, which did
not have the intended effect of making VBR mode behave like CBR mode,
but always calculated an offset equal to the selected start time.