Improve text rendering in light mode

[emilk]

This changes how we convert glyph coverage to alpha (and ultimately a color), but only in light mode.

This is a bit of a hack, because it doesn't fix dark-on-light text in dark mode (if you have any), but for the common case this PR is a huge improvement.

You can also tweak this yourself now using Visuals::text_alpha_from_coverage or from the UI (bottom of the image):

Before / After

Black text Before/after

If you think the text above looks too weak, it's only because of the default text color. Here's how it looks like with perfectly #000000 black text:

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/7290-emilkimprove-light-mode-text
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

[Wumpf]

If nothing else it's now a lot easier to understand what's happening and why, nice!!

Tested the looks on low-dpi SDR Windows Firefox: I found the default settings a tad bit too light, empirically a gamma of 0.9 looked a lil bit better to me. But this is starting to get arbitrary so if you think linear looks good I'm very much in favor with sticking with that
Either way I also believe this to be an improvement compared the harsh and fringy looking boundaries of TwoCoverageMinusCoverageSq