tick method
Called each time there is an update from the ticker on the AnimatedBackground
The implementation must return true if there is a need to repaint and false otherwise.
Implementation
@override
bool tick(double delta, Duration elapsed) {
if (_flying) {
_childZ = math.max(0.0, _childZ - 50 * delta);
renderObject!.markNeedsLayout();
if (_childZ == 0.0) _flying = false;
}
return super.tick(delta, elapsed);
}