_getColor(imageData, x, y) {
      const color = [0, 0, 0, 0];
      const { data, width } = imageData;
      const bytes = 4;
      const position = (width * y + x) * bytes;

      color[0] = data[position];
      color[1] = data[position + 1];
      color[2] = data[position + 2];
      color[3] = data[position + 3];

      return color;
    }
Get color at (x, y)
@param {Object} imageData of canvas
@param {Number} x left position
@param {Number} y top position
@returns {Array} color array