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.