"You don't end up with a tidy hierarchy this way, but I'm not sure you'd end up with a tidy hierarchy using inheritance, either."

Well, I am sure, that all the graphic libaries I ever used, had this inheritance model. (The graphics libary I build, as well.)

The libaries I have seen, that used a different model, I did not really like and they were also rather exotic, than in wide use. But I am willing to take a look at better designed succesful inheritance free ones, to see how it can be done,.if you happen to know one ..