Account for the fact that touch devices have no mouse and therefore no last mouse position.  When "undo" is invoked in a mobile app, sync the meaningless last-mouse-position point to the last point in the digitized linestring so there is visible feedback that something has been undone.  Otherwise the last-mouse-position point is left at the point of the last tap on the map, which is unfortunately the location of the vertex that has just been undone, incorrectly implying that the vertex is still there.
Merge pull request #927 from adv-isu/HandlerPathUndoTouchFix2

Improved touch support for Handler.Path.undo