Add Transform2D struct

[yoanlcq]

Add this kind of data structure :

pub struct Transform2D<P,O,S> {
    pub position: Vec2<P>,
    pub rotation_z: O, // In degrees
    pub scale: Vec2<S>,
}

If a z position element is needed, people should just use Transform instead.
The reason is, we shouldn't enforce the way to set z-order. Some architectures might be "layer-based", for instance.