Fix handling of touchmove events on Android
Fix handling of touchmove events on Android

Dragging the canvas did not work on Android devices as the === check for
lastTouch and the current event was always evaluating to false.
Presumably Safari on iOS re-uses the same Touch object for touchmove
events with the same finger, whereas Chrome/Firefox on Android creates
new Touch objects for each event (so the === evaluates false). The code
now compares Touch.identifier to ensure the new touch event is from the
same finger as the initiating touchstart.

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