libs/gst/controller/: Save last synced value from the list to continue searching...

libs/gst/controller/: Save last synced value from the list to continue searching from there in future syncs. This spe...

Original commit message from CVS:
reviewed by: Stefan Kost <ensonic@users.sf.net>
* libs/gst/controller/gstcontroller.c: (gst_controller_unset),
(gst_controller_unset_all):
* libs/gst/controller/gstcontrollerprivate.h:
* libs/gst/controller/gstinterpolation.c:
(gst_controlled_property_find_control_point_node):
Save last synced value from the list to continue searching from there
in future syncs. This speeds everything up a bit.