Called whenever the panels' layout needs to be performed again. This means all child panels must be re-positioned to fit the possibly new size of this panel.
This can be triggered in numerous ways:
- Panel:InvalidateLayout was called this or previous frame (depending on the argument)
- A child element was added to this panel (TODO: Verify me)
- The size of this panel has changed
Do NOT call this function directly. Use Panel:InvalidateLayout instead!