Optimize: get glyph uv rects during layouts instead of in tesselation

This allows them to be cached, saving around 20% total CPU.

It also makes the code more nicely structured

epaint/src/text/fonts.rs

@@ -247,6 +247,7 @@ impl Fonts {
         }
     }
 
+    #[inline(always)]
     pub fn pixels_per_point(&self) -> f32 {
         self.pixels_per_point
     }
@@ -255,6 +256,11 @@ impl Fonts {
         &self.definitions
     }
 
+    #[inline(always)]
+    pub fn round_to_pixel(&self, point: f32) -> f32 {
+        (point * self.pixels_per_point).round() / self.pixels_per_point
+    }
+
     /// Call each frame to get the latest available font texture data.
     pub fn texture(&self) -> Arc<Texture> {
         let atlas = self.atlas.lock();