Change Ellipse representation and improve helpers

[Jondolf]

Objective

Currently, the Ellipse primitive is represented by a half_width and half_height. To improve consistency (similarly to #11434), it might make more sense to use a Vec2 half_size instead.

Alternatively, to make the elliptical nature clearer, the properties could also be called radius_x and radius_y.

Secondly, Ellipse::new currently takes a full width and height instead of two radii. I would expect it to take the half-width and half-height because ellipses and circles are almost always defined using radii. I wouldn't expect Circle::new to take a diameter (if we had that method).

Solution

Change Ellipse to store a half_size and new to take the half-width and half-height.

I also added a from_size method similar to Rectangle::from_size, and added the semi_minor and semi_major helpers to get the semi-minor/major radius.

[NiseVoid]

The improved consistency is nice, more helpers is good too