gst/videomixer/videomixer.c: Fix memleak by unref'ing collectpads instance (when...

gst/videomixer/videomixer.c: Fix memleak by unref'ing collectpads instance (when finalizing)

Original commit message from CVS:
Patch by: Mark Nauwelaerts  <manauw at skynet be>
* gst/videomixer/videomixer.c:
(gst_videomixer_set_master_geometry),
(gst_videomixer_pad_sink_setcaps), (gst_videomixer_class_init),
(gst_videomixer_collect_free), (gst_videomixer_reset),
(gst_videomixer_init), (gst_videomixer_finalize),
(gst_videomixer_request_new_pad), (gst_videomixer_release_pad),
(gst_videomixer_collected), (gst_videomixer_change_state):
Fix memleak by unref'ing collectpads instance (when finalizing)
Implement releasing a request pad. Fixes #374479.