IFS fractals (the Barnsley fern and friends) drawn with the chaos game

Draws fractals defined by iterated function systems (IFS) in an OCaml Graphics window, or renders them to PNG files (optionally colored by point density, and with no display needed), using the chaos game. Sixteen fractals are predefined (the Barnsley fern, the Sierpinski triangle, a dragon curve, a maple leaf, a sunflower, Queen Anne's lace, ...), available both from the ifs-fractals command-line tool and as a library for the toplevel. Defining a new fractal is a small record: a handful of affine transforms with their weights and a viewport.