_prepareNewFrame() {
        var viewportSize = this._calculateCanvasSize();
        if( this.canvas.width !== viewportSize.x ||
            this.canvas.height !== viewportSize.y ) {
            this.canvas.width = viewportSize.x;
            this.canvas.height = viewportSize.y;
            this._updateImageSmoothingEnabled(this.context);
            if ( this.sketchCanvas !== null ) {
                var sketchCanvasSize = this._calculateSketchCanvasSize();
                this.sketchCanvas.width = sketchCanvasSize.x;
                this.sketchCanvas.height = sketchCanvasSize.y;
                this._updateImageSmoothingEnabled(this.sketchContext);
            }
        }
        this._clear();
    }
Clears the Drawer so it's ready to draw another frame.
@private