SetPositioning exposition in the API

The method in ol.Overlay is not exposed whereas is equivalent
getter getPositioning is visible in API and documentation.
The PR add a line of description to the documentation and expose
the method
Align pixels when drawing icons or shapes

The floor before setting the transform ensures that we translate by integer pixels for odd and even dimenstions.  The round before drawing the image ensures pixels are aligned.
Fix a bug when frameState is null which occurs in offline mobile devices
Add new condition param to ol.control.DragBox
snapshot