videorate: Don't forget last_ts on caps changes

videorate: Don't forget last_ts on caps changes

Whenever that caps changes does not imply that a new segment will start.
Don't reset the last_ts if only the caps have changed. This fixes issues
if you have a stream without only first frame with TS=0, and have resolution
change happening. This was a regression introduced by !3059, which issue was
described in #1352. The reported issue is still fix after this change.

Fixes #1034

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5712>