OpenLayers Undo/Redo Drawing Methods

# Undo/Redo Drawing

Demonstrates the use of undo & redo methods while drawing.

Use `Ctrl-Z` or `⌘-Z` to undo while drawing. Use `Ctrl-Y` or `⌘-Y` to redo what you have undone. Use `Esc` to cancel the current sketch.

The `control.undo` method works on the current sketch, removing the most recently added point. The `control.redo` method adds back items that were removed from an undo. To fully erase a sketch, call the `control.cancel` method.

View the draw-undo-redo.js source to see how this is done.