Virtual nodes are placed with low precision

See example at http://share.mapbbcode.org/blrtr/ldkeg : zoom in to a segment and see where vritual (semi-transparent) markers are.

The bug is in [this method](https://github.com/Leaflet/Leaflet.draw/blob/master/src/edit/handler/Edit.Poly.js#L235): layer point coordinates are integers, and when zoom is low, integer operations are imprecise.
