Fix an alignment issue with the null renderer.
Fix an alignment issue with the null renderer.

At non-integer zoom levels, a pan would result in the tiles being offset from where they should have been.

I'm not sure how to test this short of using image comparison.  We could make a test with tile layers using different renderers of different opacities, so that they should line up after panning or other actions.

Fixes issue #575.

Kitware/geojs,OpenGeoscience/geojs,Kitware/geojs,OpenGeoscience/geojs,OpenGeoscience/geojs,Kitware/geojs