gst/videomixer/videomixer.*: Implement proper seek/newsegment handling.

Original commit message from CVS:
* gst/videomixer/videomixer.c: (gst_videomixer_reset),
(gst_videomixer_init), (gst_videomixer_request_new_pad),
(gst_videomixer_fill_queues), (forward_event_func),
(forward_event), (gst_videomixer_src_event),
(gst_videomixer_sink_event):
* gst/videomixer/videomixer.h:
Implement proper seek/newsegment handling.
Based on adder's implementation.
Fixes #535121
3 files changed