_calculateCanvasSize() {
        var pixelDensityRatio = $.pixelDensityRatio;
        var viewportSize = this.viewport.getContainerSize();
        return new OpenSeadragon.Point( Math.round(viewportSize.x * pixelDensityRatio), Math.round(viewportSize.y * pixelDensityRatio));
    }
Calculate width and height of the canvas based on viewport dimensions
and pixelDensityRatio
@private
@returns {OpenSeadragon.Point} {x, y} size of the canvas