def []=(x, y, color)
      assert_xy!(x, y)
      @pixels[y * width + x] = ChunkyPNG::Color.parse(color)
    end
def []=(x, y, color)
      assert_xy!(x, y)
      @pixels[y * width + x] = ChunkyPNG::Color.parse(color)
    end
Replaces a single pixel in this canvas.
 @param [Integer] x The x-coordinate of the pixel (column)
 @param [Integer] y The y-coordinate of the pixel (row)
 @param [Integer] color The new color for the provided coordinates.
 @return [Integer] The new color value for this pixel, i.e.
   <tt>color</tt>.
 @raise [ChunkyPNG::OutOfBounds] when the coordinates are outside of the
   image's dimensions.
 @see #set_pixel