A test render.

Screenshot from the editor, along with the test render. Concept of user interface: the scene is represented as tree of "operators", operator tree is displayed on the left, operator properties on the right. Operators have channels for connections between them; the Perlin noise operator's evaluation position and result are connected to Monofractal, which in turn is connected to clouds. There's some usability problem: it's possible to get noise connected directly to clouds, bypassing the Monofractal. I'm unsure how to go about this usability problem.
The most important lacking thing now is curve editor, it is necessary for getting clouds disappear at top and bottom of layer rather than get clipped by layer bounds.