gst/videomixer/videomixer.c: Implement GstChildProxy interface.

gst/videomixer/videomixer.c: Implement GstChildProxy interface.

Original commit message from CVS:
reviewed by: Edward Hervey  <edward.hervey@collabora.co.uk>
* gst/videomixer/videomixer.c:
(gst_videomixer_set_master_geometry), (_do_init),
(gst_videomixer_child_proxy_get_child_by_index),
(gst_videomixer_child_proxy_get_children_count),
(gst_videomixer_child_proxy_init), (gst_videomixer_reset),
(gst_videomixer_init), (gst_videomixer_request_new_pad),
(gst_videomixer_release_pad), (gst_videomixer_fill_queues):
Implement GstChildProxy interface.
Send newsegment at the right moment
Fixes #488879