Optimize tessellation of filled circles (#1616)

When painting a scatter plot we sometimes want to paint hundreds of thousands of points (filled circles) on screen every frame.

In this PR the font texture atlas is pre-populated with some filled circled of various radii. These are then used when painting (small) filled circled, which means A LOT less triangles and vertices are generated for them.

In a new benchmark we can see a 10x speedup in circle tessellation, but the the real benefit comes in the painting of these circles: since we generate a lot less vertices, the backend painter has less to do.

In a real-life scenario with a lot of things being painted (including around 100k points) I saw tessellation go from 35ms -> 7ms and painting go from 45ms -> 1ms. This means the total frame time went from 80ms to 8ms, or a 10x speedup.

egui_demo_lib/benches/benchmark.rs

@@ -124,8 +124,13 @@ pub fn criterion_benchmark(c: &mut Criterion) {
 
         let galley = fonts.layout(LOREM_IPSUM_LONG.to_owned(), font_id, color, wrap_width);
         let font_image_size = fonts.font_image_size();
-        let mut tessellator =
-            egui::epaint::Tessellator::new(1.0, Default::default(), font_image_size);
+        let prepared_discs = fonts.texture_atlas().lock().prepared_discs();
+        let mut tessellator = egui::epaint::Tessellator::new(
+            1.0,
+            Default::default(),
+            font_image_size,
+            prepared_discs,
+        );
         let mut mesh = egui::epaint::Mesh::default();
         let text_shape = TextShape::new(egui::Pos2::ZERO, galley);
         c.bench_function("tessellate_text", |b| {