Move ImageAlphaType into being ImageFormats instead

[DJMcNab]

I'm not planning to merge this even with an approval, without some more discussion.
Just exploring what this would look like practically.

See the discussion in #121 (review).

[nicoburns]

It may make sense to make ImageFormat a custom bit field where "is_premul" has a dedicated bit

[DJMcNab]

It may make sense to make ImageFormat a custom bit field where "is_premul" has a dedicated bit

I think the ergonomics of that are pretty poor (you lose exhaustive matching), but I can see the appeal.

Fwiw, as it stands, the least significant bit is effectively the "is_premul" bit.

[sagudev]

Alternative approach:

enum ImageFormat {
    Rgba8(AlphaType),
    Bgra8(AlphaType),
}

pros:

1. keeps orthogonality of types

cons:

1. ergonomics (nested type)

And TBH I think the ergonomics of nested type (easiness of use) is bothering me, so I am not really sure that this alternative approach is anyway better.