instance ColorConvertible PixelYA8 PixelRGB8 loses data

[nomeata]

According to the docs, promotePixel should not lose any data, but yet

instance ColorConvertible PixelYA8 PixelRGB8 where
    {-# INLINE promotePixel #-}
    promotePixel (PixelYA8 y _) = PixelRGB8 y y y

Maybe the instance should receive a haddock comment warning about this, or the docs of promotePixel should indicate that alpha channel information can be lost.

[Twinside]

Clearly inconsistent, I'd like to remove the instance, but bumping the major version number for this tiny case bug me a little.

There is a need to think about it.

[nomeata]

I actually make use of the instance now... but I can work around its removal.

[Twinside]

I also found myself abusing it in the test cases, but i guess adding a deprecated pragma is way to go with a bump of the documentation