ol-ext: interaction touch draw

ol.interaction.TouchDraw is an interaction to use on touch devices. The pointer is deferred to the center of the viewport and a target is drawn to materialize this point.
So drawing consists in moving the map under the target thus the sketch is not masked by the finger when moving. It derives from an ol.interaction.CenterTouch that handle the centering: The example uses an ol.control.Bar with ol.control.Toggle to control the interaction.