hg clone git+https://anongit.kde.org/krita
Well, the @krita devs seem to think that a generalised wallpaper function would be interesting. Let me see what I can do.
Whoa. Building #Krita was absolutely painless. I saw CMake files, did the usual thing, waited a few minutes, and WHOOP out popped a Krita.
Guess I can start reading its source code now.