Returns a rect of the given size, centered at this fraction of the given rect.
For example, a 100×100 size inscribed on a 200×200 rect using FractionalOffset.topLeft would be the 100×100 rect at the top left of the 200×200 rect.
Source
Rect inscribe(Size size, Rect rect) {
return new Rect.fromLTWH(
rect.left + (rect.width - size.width) * dx,
rect.top + (rect.height - size.height) * dy,
size.width,
size.height
);
}