I would like to be able to extract an alpha channel as a separate luma image.

My specific use case for this functionality is that some formats like PDF need alpha as a separate image called "softmask".

More generally it could be possible to extract any combination of channels like just get RGB from RGBA or individual channels like just G channel as luma image. It is usable that this is part of this crate as outside code would need to guard every time a new image format is added into DynamicImage enum which is non exhaustive.