clip: be more robust in handling priority

clip: be more robust in handling priority

Make less assumptions about the priority of effects and core elements so
that the code would still work if the priority of an element was set
directly. In particular, the index of a top effect will always be its
position in the effect ordering.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/merge_requests/169>