/**返回指定点alpha像素值。
* Returns alpha pixel value at the specified point.
*
* @method getAlphaPixel
* @param {Number} x The x coordinate of the pixel in the bitmap
* @param {Number} y The y coordinate of the pixel in the bitmap
* @return {String} The alpha value
*/
Bitmap.prototype.getAlphaPixel = function(x, y) {
var data = this._context.getImageData(x, y, 1, 1).data;
return data[3];
};