JSI_IGUIObject should not contain functions that work only for some GUI Object types, refs #5442.
Deduplicate the shuffled copy of CText::SetupText.
Avoid the 80 times slower dynamic_cast mandated by the virtual class inheritance by adding an ugly overridable pointer to the base class pointing at this derived classes, as bargained with Vladislav and proposed by wraitii in D1781 id=8426.
This may be improved by refactoring the IGUIObject and JSInterface classes to use templates and / or eliminating its virtual inheritance.
Implement and use FromJSVal / ToJSVal CSize specialization.
Remove the JS::CallArgsFromVp call.
Differential Revision: https://code.wildfiregames.com/D2136
Comments By: wraitii, Vladislav