Add "explore" support for Barnsley Fern

[MatthewPeterKelly]

Pan and zoom don't make much sense on this one -- it would be expensive and not show too much new detail (it's perfectly self similar all the way down).

Instead, connect the keys to adjusting the parameters that control the shape of the fern.

[MatthewPeterKelly]

There are 27 independent shape parameters... more like 19 if you lock in the sparsity pattern. This is too many to easily hook up to the keyboard.

Unless... we follow the lessons of stochastic gradient descent and have one key pick a set of random directions and then use, for example, the arrow keys to manually do a line search along the selected gradient. Then sample new directions as needed.

[MatthewPeterKelly]

You could even maintain an ordered list of search directions, and allow the user to traverse back and forth along that list.