_raiseTileDrawingEvent(tiledImage, context, tile, rendered){
        /**
         * This event is fired just before the tile is drawn giving the application a chance to alter the image.
         *
         * NOTE: This event is only fired when the 'canvas' drawer is being used
         *
         * @event tile-drawing
         * @memberof OpenSeadragon.Viewer
         * @type {object}
         * @property {OpenSeadragon.Viewer} eventSource - A reference to the Viewer which raised the event.
         * @property {OpenSeadragon.Tile} tile - The Tile being drawn.
         * @property {OpenSeadragon.TiledImage} tiledImage - Which TiledImage is being drawn.
         * @property {CanvasRenderingContext2D} context - The HTML canvas context being drawn into.
         * @property {CanvasRenderingContext2D} rendered - The HTML canvas context containing the tile imagery.
         * @property {?Object} userData - Arbitrary subscriber-defined object.
         */
        this.viewer.raiseEvent('tile-drawing', {
            tiledImage: tiledImage,
            context: context,
            tile: tile,
            rendered: rendered
        });
    }
Fires the tile-drawing event.
@private