The most important thing to make it work is actually notifying the scene that something has changed -- the rest are just refactorings and (hopefully) optimizations.
part of CURA-12543
Should be able to paint pixels now if the tools is active, and the model loaded is with UV-coords (that rules out our current impl. of 3MF at the moment -- use OBJ instead), and you position the model outside of the build-plate so the paint-shadr that is temporarily replacing the 'disabled' one is showing.
Will need a lot of extra features and optimizations still!
part of CURA-12543
