code: function CanvasSurface(options) {
      if (options)
          options.useTarget = true;
      else
          options = { useTarget: true };
  
      if (options.canvasSize) this._canvasSize = options.canvasSize;
      Surface.apply(this, arguments);
      if (!this._canvasSize) this._canvasSize = this.getSize();
      this._backBuffer = document.createElement('canvas');
      if (this._canvasSize) {
          this._backBuffer.width = this._canvasSize[0];
          this._backBuffer.height = this._canvasSize[1];
      }
      this._contextId = undefined;
  }
docs: A surface containing an HTML5 Canvas element.
  This extends the Surface class.
  
  @class CanvasSurface
  @extends Surface
  @constructor
  @param {Object} [options] overrides of default options
  @param {Array.Number} [options.canvasSize] [width, height] for document element