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

epaint/src/text/font.rs

@@ -377,7 +377,7 @@ fn allocate_glyph(
                 if v > 0.0 {
                     let px = glyph_pos.0 + x as usize;
                     let py = glyph_pos.1 + y as usize;
-                    image[(px, py)] = fast_round(v * 255.0);
+                    image[(px, py)] = v;
                 }
             });
 
@@ -405,7 +405,3 @@ fn allocate_glyph(
         uv_rect,
     }
 }
-
-fn fast_round(r: f32) -> u8 {
-    (r + 0.5).floor() as _ // rust does a saturating cast since 1.45
-}

epaint/src/text/fonts.rs

@@ -539,7 +539,7 @@ impl FontsImpl {
             // Make the top left pixel fully white:
             let (pos, image) = atlas.allocate((1, 1));
             assert_eq!(pos, (0, 0));
-            image[pos] = 255;
+            image[pos] = 1.0;
         }
 
         let atlas = Arc::new(Mutex::new(atlas));