track-element: use out-point for updating control bindings

track-element: use out-point for updating control bindings

The out-point, which is an internal time, is used instead of the
duration for determining the control binding value at the end of the
element.

Also, allow the user to switch off the auto-clamping of control sources
if they are not desired. And allow them to clamp specific control sources
individually.

Also, fix a lot of memory leaks related to control sources. In
particular, releasing the extra ref gained by source in
g_object_get (binding, "control-source", &source, NULL);

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/merge_requests/177>