I decided to postpone turbulence filter for a while and go ahead with my original plans. So now, Inkscape has a new filter primitive, feComposite.
Composite in this case refers to Porter-Duff compositing operators, introduced in a paper "Compositing Digital Images" by T. Porter and T. Duff. These compositing operators can provide quite interesting results, though I'm yet to come up with a example where they could be used. Well, one thing useful could be cutting holes to objects, for which using boolean operations would not be feasible.
Reading that paper, so I could implement the filter, a strange feeling of perspective crossed my mind. Even though computer graphics is a rather new area of study, this paper has been written before I was even born. Only a year before, but still.
Creating this filter I came upon several parts of filtering code, that should be simpler to use. Especially handling temporary images requires too much boilerplate code. So next, I will look into improving handling temporary images.