Update me.Vector2d methods to return a reference to `this` for method chaining.

Example:
var vec = new me.Vector2d(
    Math.cos(Math.PI / 4),
    Math.sin(Math.PI / 4)
).scale(new me.Vector2d(16, 16)).floorSelf().negateSelf();

- Creates a new unit vector at 45 degrees.
- Scales the vector to 16 units.
- Floors the product.
- Negates the result.