[GSoC 2023] Next up: Add reference images

I have fixed three memory leaks. Before, leak_analyser.sh found 75 leaks from simply opening and closing Icon-O-Matic. It now reports only 27.

I am now planning to implement a new shape type called “reference images”. This implements ticket #2748. As discussed in the ticket, having a reference image in the background greatly assists in vectorizing it. Reference images are just for reference. They will not be exported to the final HVIF file.