clip: Fix track element priority computation

clip: Fix track element priority computation

We were computing the priority offset taking the global MIN_NLE_PRIO
(which is a constant == 2 to make space for the mixing elements) instead
of the layer 'track element' relative priority, leading to very big
offsets on layer with a prio > 0. In the end it leaded to effects having
the same priority as the sources which leads to an undefined behaviour
in NLE.