Make the font atlas use a color image

[valadaptive]

• I have followed the instructions in the PR template

Splitting this out from the Parley work as requested. This removes FontImage and makes the font atlas use a ColorImage. It converts alpha to coverage at glyph-drawing time, not at delta-upload time.

This doesn't do much now, but will allow for color emoji rendering once we start using Parley.

I've changed things around so that we pass in text_alpha_to_coverage to the Fonts the same way we do with pixels_per_point and max_texture_side, reusing the existing code to check if the setting differs and recreating the font atlas if so. I'm not quite sure why this wasn't done in the first place.

I've left ImageData as an enum for now, in case we want to add support for more texture pixel formats in the future (which I personally think would be worthwhile). If you'd like, I can just remove that enum entirely.

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/7298-color-font-atlas
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

[emilk]

This is perfect; thank you!