Handle simultaneous touch events
Handle simultaneous touch events

Found and tested on an iPhone 5s w/ iOS 9.2.
Not sure about other devices.

Fixes #877

seanich/openseadragon,avandecreme/openseadragon,jmnetus/openseadragon,openseadragon/openseadragon,LarissaSmith/openseadragon,avandecreme/openseadragon,msalsbery/openseadragon,rdlester/openseadragon,seanich/openseadragon,msalsbery/openseadragon,openseadragon/openseadragon,LarissaSmith/openseadragon,jmnetus/openseadragon,rdlester/openseadragon