When available, use the mouse event buttons property.
When available, use the mouse event buttons property.

Firefox erroneously sets the which value to the left button even when it
is not held down.  The buttons property is correct for browsers that
support it, but Safari doesn't, so we have to fall back to using the
which value in that case.

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