function unmultiply (rgb, alphaData) {
		for (var i = 3, len = rgb.data.length; i < len; i = i + 4) {
			rgb.data[i] = alphaData[i - 1]; //copy B value into A channel
			rgb.data[i - 3] = rgb.data[i - 3] / (alphaData[i - 1] ? (alphaData[i - 1] / 255) : 1); //un-premultiply B
			rgb.data[i - 2] = rgb.data[i - 2] / (alphaData[i - 1] ? (alphaData[i - 1] / 255) : 1); //un-premultiply G
			rgb.data[i - 1] = rgb.data[i - 1] / (alphaData[i - 1] ? (alphaData[i - 1] / 255) : 1); //un-premultiply R
		}
		return rgb;
	}
unmultiply an image with alpha information
@param {Array} rgb - canvasPixelArray representing the image to be unmultiplied
@param {Array} alphaData - canvasPixelArray representing the alpha to use
@returns {Array} rgb