Fix buggy text withviewports on monitors with different scales (#3666)

* Closes https://github.com/emilk/egui/issues/3664

Bonus: optimize color conversions and font atlas upload, especially in
debug builds.

crates/epaint/src/image.rs

@@ -350,7 +350,7 @@ impl From<FontImage> for ImageData {
 
 #[inline]
 fn fast_round(r: f32) -> u8 {
-    (r + 0.5).floor() as _ // rust does a saturating cast since 1.45
+    (r + 0.5) as _ // rust does a saturating cast since 1.45
 }
 
 // ----------------------------------------------------------------------------

crates/epaint/src/text/fonts.rs

@@ -425,6 +425,12 @@ impl Fonts {
         self.lock().fonts.atlas.clone()
     }
 
+    /// The full font atlas image.
+    #[inline]
+    pub fn image(&self) -> crate::FontImage {
+        self.lock().fonts.atlas.lock().image().clone()
+    }
+
     /// Current size of the font image.
     /// Pass this to [`crate::Tessellator`].
     pub fn font_image_size(&self) -> [usize; 2] {
@@ -590,7 +596,7 @@ impl FontsImpl {
         );
 
         let texture_width = max_texture_side.at_most(8 * 1024);
-        let initial_height = 64;
+        let initial_height = 32; // Keep initial font atlas small, so it is fast to upload to GPU. This will expand as needed anyways.
         let atlas = TextureAtlas::new([texture_width, initial_height]);
 
         let atlas = Arc::new(Mutex::new(atlas));

crates/epaint/src/texture_atlas.rs

@@ -97,7 +97,7 @@ impl TextureAtlas {
         // for r in [1, 2, 4, 8, 16, 32, 64] {
         //     let w = 2 * r + 3;
         //     let hw = w as i32 / 2;
-        const LARGEST_CIRCLE_RADIUS: f32 = 64.0;
+        const LARGEST_CIRCLE_RADIUS: f32 = 8.0; // keep small so that the initial texture atlas is small
         for i in 0.. {
             let r = 2.0_f32.powf(i as f32 / 2.0 - 1.0);
             if r > LARGEST_CIRCLE_RADIUS {
@@ -172,9 +172,21 @@ impl TextureAtlas {
         }
     }
 
+    /// The texture options suitable for a font texture
+    #[inline]
+    pub fn texture_options() -> crate::textures::TextureOptions {
+        crate::textures::TextureOptions::LINEAR
+    }
+
+    /// The full font atlas image.
+    #[inline]
+    pub fn image(&self) -> &FontImage {
+        &self.image
+    }
+
     /// Call to get the change to the image since last call.
     pub fn take_delta(&mut self) -> Option<ImageDelta> {
-        let texture_options = crate::textures::TextureOptions::LINEAR;
+        let texture_options = Self::texture_options();
 
         let dirty = std::mem::replace(&mut self.dirty, Rectu::NOTHING);
         if dirty == Rectu::NOTHING {