Constructive geometry

Additional operations should be added to combine shapes:

```
intersection :: [Picture] -> Picture
difference :: Picture -> Picture -> Picture
```

(Names are debatable.) `intersection` should produce a picture bounded by the intersection of the list. The second should produce a picture containing all areas of the first image that are not also in the second. Note that together with the existing `pictures`, this gives the three basic operations for constructive geometry.

The implementation is a little tricky. Combining two images in this way can be done with globalCompositeOperation (https://developer.mozilla.org/samples/canvas-tutorial/6_1_canvas_composite.html) But getting the resulting shapes to nest correctly will involve using off-screen canvases as temporaries. Since creating a new off-screen canvas is expensive, this should involve determining the minimum number of offscreen canvases needed, and reusing them between frames.