Add similar refPoint validity check to zoomBy
Add similar refPoint validity check to zoomBy

lidasong2014/openseadragon,janelia-flyem/openseadragon,janelia-flyem/openseadragon,lidasong2014/openseadragon,avandecreme/openseadragon,goofwear/openseadragon,LafayetteCollegeLibraries/openseadragon,ricochet2200/openseadragon,mcanthony/openseadragon,seanich/openseadragon,shaunstanislaus/openseadragon,jmnetus/openseadragon,msalsbery/openseadragon,rdlester/openseadragon,goofwear/openseadragon,Iced-Tea/openseadragon,Iced-Tea/openseadragon,msalsbery/openseadragon,LarissaSmith/openseadragon,daverome/openseadragon,TMUND/openseadragon,seanich/openseadragon,TMUND/openseadragon,openseadragon/openseadragon,mcanthony/openseadragon,daverome/openseadragon,ricochet2200/openseadragon,avandecreme/openseadragon,openseadragon/openseadragon,rdlester/openseadragon,shaunstanislaus/openseadragon,sul-dlss/openseadragon,LarissaSmith/openseadragon,jmnetus/openseadragon