Explore dithered alpha frame constructor

[197g]

When we create a palette for an image from RGBA8 and LA8 we use alpha > 0 as a hard-coded condition for transparency. There's of course alternatives for this such as alpha > 127 etc. In the end, these functions aim at good approximation with practical compute costs. We could offer alternative constructors. In particular, dithering is an solution space for matching colors with a limited palette, maybe we could use a similar algorithm to dither transparency such that the total transparency over an area roughly matches the underlying transparency.

Solutions should be well-defined enough to be maintainable, i.e. have clear usage guidelines and documented characteristics.

Raised here: https://github.com/image-rs/image-gif/pull/223#discussion_r2551364747

[Shnatsel]

alpha > 127 is what I need for wondermagick: https://github.com/Shnatsel/wondermagick/issues/59

I could also work around that in alpha adjustment prepass at the cost of some performance.