Add opacity factor to TextShape (#3916)

<!--
Please read the "Making a PR" section of
[`CONTRIBUTING.md`](https://github.com/emilk/egui/blob/master/CONTRIBUTING.md)
before opening a Pull Request!

* Keep your PR:s small and focused.
* If applicable, add a screenshot or gif.
* If it is a non-trivial addition, consider adding a demo for it to
`egui_demo_lib`, or a new example.
* Do NOT open PR:s from your `master` branch, as that makes it hard for
maintainers to add commits to your PR.
* Remember to run `cargo fmt` and `cargo cranky`.
* Open the PR as a draft until you have self-reviewed it and run
`./scripts/check.sh`.
* When you have addressed a PR comment, mark it as resolved.

Please be patient! I will review your PR, but my time is limited!
-->

This PR simply allows you to override the opacity of a Galley when you
draw it on screen. Last year I opened #3548 and some changes were
requested to the PR, but unfortunately school got really busy and I
wasn't able to apply them. This PR supersedes #3548 and applys the
changes requested in that PR

Closes #3548

crates/epaint/src/shape_transform.rs

@@ -56,6 +56,7 @@ pub fn adjust_colors(shape: &mut Shape, adjust_color: &impl Fn(&mut Color32)) {
             underline,
             fallback_color,
             override_text_color,
+            opacity_factor: _,
             angle: _,
         }) => {
             adjust_color(&mut underline.color);