Reset to "start drawing" mode

[danp]

Once the freedraw param has a value, there's no way to grab the map and move it around: clicking and dragging adds new lines.

The param value can be cleared but this resets the zoom and such.

It would be nice if there was a way to reset back to how things work when the page is first loaded, the param is empty, the "start drawing" button is visible, and the map can be moved without drawing.