add back gravity for green circle in demo
add back gravity for green circle in demo

TheGuardianWolf/OpenSeadragonPaperjsOverlay,eriksjolund/OpenSeadragonPaperjsOverlay,TheGuardianWolf/OpenSeadragonPaperjsOverlay,eriksjolund/OpenSeadragonPaperjsOverlay,altert/OpenseadragonFabricjsOverlay,altert/OpenseadragonFabricjsOverlay