message: Consistent definition of Rgba
  Proposing a change to make `Rgba` type definition similar to how `LumaA` is defined. It makes it easier to distinguish the different with `Rgb` type.
diff: @@ -53,7 +53,7 @@ The most important methods for decoders are...
  `image` provides the following pixel types:
  + **Rgb**: RGB pixel
  -+ **Rgba**: RGBA pixel
  ++ **Rgba**: RGB with alpha (RGBA pixel)
  + **Luma**: Grayscale pixel
  + **LumaA**: Grayscale with alpha
  