implementation of #16

If we're going to treat the browser as our wall, we should reflect its difference from a wall.

How are we to act when directed to draw 12 inch lines? Let us ask our audience to guide us. We are working in an interactive medium.

Rather than being an execution of `Wall Drawing #16`, this code is a generator over the space of many possible interpretations of same.

It has three operations:

* clicking changes the point where the three lines intersect
* clicking and dragging in the Y direction changes the number of lines in each band, which can vary from 4 to 200
* clicking and dragging in the X direction changes the space between the lines in each band, anywhere from 2 to 20 pixels.

It also has a small legend with instructions, which remains hidden until the user rolls over the question mark.