Improve text contrast in bright mode (#1412)

* Rename AlphaImage to FontImage to discourage any other use for it
* Encode FontImage as f32 and postpone the alpha correction
* Interpret alpha coverage in a new, making dark text darker, improving contrast in bright mode

egui/src/context.rs

@@ -17,7 +17,7 @@ impl Default for WrappedTextureManager {
         // Will be filled in later
         let font_id = tex_mngr.alloc(
             "egui_font_texture".into(),
-            epaint::AlphaImage::new([0, 0]).into(),
+            epaint::FontImage::new([0, 0]).into(),
         );
         assert_eq!(font_id, TextureId::default());
 

egui/src/lib.rs

@@ -307,7 +307,7 @@ pub use epaint::{
     color, mutex,
     text::{FontData, FontDefinitions, FontFamily, FontId, FontTweak},
     textures::TexturesDelta,
-    AlphaImage, ClippedPrimitive, Color32, ColorImage, ImageData, Mesh, PaintCallback,
+    ClippedPrimitive, Color32, ColorImage, FontImage, ImageData, Mesh, PaintCallback,
     PaintCallbackInfo, Rgba, Rounding, Shape, Stroke, TextureHandle, TextureId,
 };