gst/videomixer/videomixer.c: Use stream_time to synchronize the object properties.
Original commit message from CVS: * gst/videomixer/videomixer.c: (gst_videomixer_fill_queues), (gst_videomixer_blend_buffers): Use stream_time to synchronize the object properties. Use running_time of the master pad to timestamp outgoing buffers. Fix the initial segment event to extend an unknown amount of time. Fixes #537361.