videomixer: use gst_util_uint64_scale*_round.

videomixer: use gst_util_uint64_scale*_round.

There could be a case where:
      1) you do a new set_caps after buffers have been processed.
      2) ts_offset gets set to a different value, eg 0.033333333
      3) your pads get EOS, but the check dor that doesn't work
         because you use ts_offset + a truncated value < segment.stop
      4) so in the next collected, you end up comparing for example:
      0.9999999999 > 1., which is false and means you don't send EOS.

Also adds scale_round in two other places where it potentially could
have caused problems.