GstAudioEncoder: Do not drain and reset on new segment

It causes PTS and DTS to restart from 0 because priv->base_ts gets reset
to GST_CLOCK_TIME_NONE. Downstream, at least mpegtsmux, would then print
"ignoring DTS going backward" warning in loop.

GstVideoEncoder and GstAudioDecoder are not doing anything special on
SEGMENT event, so this is at least more consistent.

Fixes: #1237
26 jobs for !2493 with main in 35 seconds (queued for 1 second)
merge request