def decompose_alpha_components(color, mask, bg)
      [
        decompose_alpha_component(:r, color, mask, bg),
        decompose_alpha_component(:g, color, mask, bg),
        decompose_alpha_component(:b, color, mask, bg),
      ]
    end
def decompose_alpha_components(color, mask, bg)
      [
        decompose_alpha_component(:r, color, mask, bg),
        decompose_alpha_component(:g, color, mask, bg),
        decompose_alpha_component(:b, color, mask, bg),
      ]
    end
Decomposes the alpha channels for the r, g and b color channel.
 @param [Integer] color The color that was the result of compositing.
 @param [Integer] mask The opaque variant of the color that was being
   composed
 @param [Integer] bg The background color on which the color was composed.
 @return [Array<Integer>] The decomposed alpha values for the r, g and b
   channels.