Snap: Fix position bug when anchor is not top-left

[projectitis]

When a snap component (e.g. SnapRectangleComponent) has an anchor set - for eaxmple, to Anchor.center - then it will not position itself correctly when a position is set.

final rect = SnapRectangleComponent(
    size: Vector2(100, 50),
    paint: Paint()..color = const Color.fromARGB(255, 25, 111, 131),
);
rect.anchor = Anchor.center;
rect.position = Vector2(100, 50); // Parent is 200, 100