epaint: Add EllipseShape (#4122)

Adds an Ellipse shape draw-able with the Painter, brings egui closer to
the SVG Specification.

I've done some optimization towards using less vertices and doing less
calculations.

~~Currently the vertices are evenly distributed.
It's possible this could be optimized further taking into account the
gradient and increasing the concentration of vertices where the change
in gradient is larger.~~


![EllipseTall](https://github.com/emilk/egui/assets/50041841/b105230c-ce68-49c1-b162-d1f066bf9d6a)

![EllipseWide](https://github.com/emilk/egui/assets/50041841/6f9106d2-75cb-4f2a-b0c2-039b3aadec86)

crates/epaint/src/shape_transform.rs

@@ -20,6 +20,12 @@ pub fn adjust_colors(shape: &mut Shape, adjust_color: &impl Fn(&mut Color32)) {
             fill,
             stroke,
         })
+        | Shape::Ellipse(EllipseShape {
+            center: _,
+            radius: _,
+            fill,
+            stroke,
+        })
         | Shape::Path(PathShape {
             points: _,
             closed: _,