clipoverlay: Make sure priorities of Textoverlays are always higher than the sources they are overlayed on

Submitted by Thibault Saunier @thiblahute

Link to original bug (#750644)

Description

Currently the user has to always make sure an overlay has a source to apply on (adding overlayclip on a layer with higher prio), and it has to make sure that it has a source to be applied on.

We should at least detect it and warn the user.