mpegtsbase: Flush more on DISCONT buffers in push mode

mpegtsbase: Flush more on DISCONT buffers in push mode

When in push mode, we want to discard all previous observations from the
mpegtspacketizer when we get a DISCONT buffer.

This avoids trying to calculate bogus timestamps (estimating them using old
PCR observations).

We only do a hard reset in push-mode. In pull-mode we still need the observations
(in order to seek properly)