Mesh support

[hecrj]

Watch in higher quality / Code

Fixes #4 and closes #32.

This PR introduces the Shape and Mesh types.

• A Shape represents a geometric figure.
• A Mesh is a set of shapes that can be drawn.

The API is quite simple!

let shape = Shape::Circle {
    center: Point::new(0.0, 0.0),
    radius: 100.0,
};

let mut mesh = Mesh::new();

mesh.stroke(shape.clone(), Color::BLACK, 2);
mesh.fill(shape, Color::WHITE);

mesh.draw(target);

[JohnDoneth]

Wow, this is awesome! 🎉

This is just what I wanted to use when prototyping some simple game concepts. I can see this also being useful for debug drawing for something like physics bodies.