viewportCoordToDrawerCoord(point) {
        var vpPoint = this.viewport.pixelFromPointNoRotate(point, true);
        return new $.Point(
            vpPoint.x * $.pixelDensityRatio,
            vpPoint.y * $.pixelDensityRatio
        );
    }
This function converts the given point from to the drawer coordinate by
multiplying it with the pixel density.
This function does not take rotation into account, thus assuming provided
point is at 0 degree.
@param {OpenSeadragon.Point} point - the pixel point to convert
@returns {OpenSeadragon.Point} Point in drawer coordinate system.