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.

epaint/src/shadow.rs

@@ -73,6 +73,7 @@ impl Shadow {
                 ..Default::default()
             },
             font_tex_size,
+            vec![],
         );
         let mut mesh = Mesh::default();
         tessellator.tessellate_rect(&rect, &mut mesh);