Don't make a freehand line when two points aren't connected

[dabreegster]

Currently when snapping, if two waypoints are disconnected in the graph, the result is a straight line between them, and the snapped property is incorrect. At minimum, this property should be set correctly. But more ideally, there should just be no result, or a clear way to indicate failure. This should be configurable, because in the ATIP sketch use case where freehand and snapped points can be mixed, a straight line is appropriate.