epaint: Add EllipseShape

[TheTacBanana]

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.

[TheTacBanana]

Points are now distributed based on an easing function
Concentrating points around tighter bends, such as if the width if much greater than the height

[emilk]

I appreciate the PR, but I wonder how far we should be taking epaint. The purpose of epaint is to support the painting needs for a GUI, not arbitrary anythings, and certainly not to be on par with SVG. Still, ellipses isn't adding too much code or complexity, so it's probably fine.

But - what are you planning on using this for?

[TheTacBanana]

Was using ellipses to draw nodes of graphs, as it looks much better than using either a large circle with label inside or a small circle with label outside

Thought that someone else might find it useful in the future

Have changed to radius, definitely makes more sense than size

[nowakf]

I just ran into this, with the same use case!