layout method
Implementation
@override
void layout(FConstraints constraints) {
final child = this.child!;
child._doLayout(.loose(.vec2(constraints.maxWidth, constraints.maxHeight)));
final w = constraints.maxWidth.isFinite ? constraints.maxWidth : child.size.x;
final h = constraints.maxHeight.isFinite ? constraints.maxHeight : child.size.y;
child.localOffset = .vec2(
(w - child.size.x) / 2,
vertical ? (h - child.size.y) / 2 : 0,
);
size = .vec2(w, h);
}