Make the font atlas use a color image (#7298)

* [x] 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.

crates/epaint/src/text/text_layout.rs

@@ -1034,11 +1034,18 @@ fn is_cjk_break_allowed(c: char) -> bool {
 
 #[cfg(test)]
 mod tests {
+    use crate::AlphaFromCoverage;
+
     use super::{super::*, *};
 
     #[test]
     fn test_zero_max_width() {
-        let mut fonts = FontsImpl::new(1.0, 1024, FontDefinitions::default());
+        let mut fonts = FontsImpl::new(
+            1.0,
+            1024,
+            AlphaFromCoverage::default(),
+            FontDefinitions::default(),
+        );
         let mut layout_job = LayoutJob::single_section("W".into(), TextFormat::default());
         layout_job.wrap.max_width = 0.0;
         let galley = layout(&mut fonts, layout_job.into());
@@ -1049,7 +1056,12 @@ mod tests {
     fn test_truncate_with_newline() {
         // No matter where we wrap, we should be appending the newline character.
 
-        let mut fonts = FontsImpl::new(1.0, 1024, FontDefinitions::default());
+        let mut fonts = FontsImpl::new(
+            1.0,
+            1024,
+            AlphaFromCoverage::default(),
+            FontDefinitions::default(),
+        );
         let text_format = TextFormat {
             font_id: FontId::monospace(12.0),
             ..Default::default()
@@ -1094,7 +1106,12 @@ mod tests {
 
     #[test]
     fn test_cjk() {
-        let mut fonts = FontsImpl::new(1.0, 1024, FontDefinitions::default());
+        let mut fonts = FontsImpl::new(
+            1.0,
+            1024,
+            AlphaFromCoverage::default(),
+            FontDefinitions::default(),
+        );
         let mut layout_job = LayoutJob::single_section(
             "日本語とEnglishの混在した文章".into(),
             TextFormat::default(),
@@ -1109,7 +1126,12 @@ mod tests {
 
     #[test]
     fn test_pre_cjk() {
-        let mut fonts = FontsImpl::new(1.0, 1024, FontDefinitions::default());
+        let mut fonts = FontsImpl::new(
+            1.0,
+            1024,
+            AlphaFromCoverage::default(),
+            FontDefinitions::default(),
+        );
         let mut layout_job = LayoutJob::single_section(
             "日本語とEnglishの混在した文章".into(),
             TextFormat::default(),
@@ -1124,7 +1146,12 @@ mod tests {
 
     #[test]
     fn test_truncate_width() {
-        let mut fonts = FontsImpl::new(1.0, 1024, FontDefinitions::default());
+        let mut fonts = FontsImpl::new(
+            1.0,
+            1024,
+            AlphaFromCoverage::default(),
+            FontDefinitions::default(),
+        );
         let mut layout_job =
             LayoutJob::single_section("# DNA\nMore text".into(), TextFormat::default());
         layout_job.wrap.max_width = f32::INFINITY;