Improve Rectangle and Cuboid consistency

[Jondolf]

Objective

The Rectangle and Cuboid primitives currently use different representations:

pub struct Rectangle {
    /// The half width of the rectangle
    pub half_width: f32,
    /// The half height of the rectangle
    pub half_height: f32,
}

pub struct Cuboid {
    /// Half of the width, height and depth of the cuboid
    pub half_extents: Vec3,
}

The property names and helpers are also inconsistent. Cuboid has half_extents, but it also has a method called from_size. Most existing code also uses "size" instead of "extents".

Solution

Represent both Rectangle and Cuboid with half_size properties.

[alice-i-cecile]

Great, thanks.