Following recent discussions with Vladislav about virtual inheritance in the context of rP22596 / D2136,
a measurement that showed that `dynamic_cast` is 80 times slower than `static_cast` to determine the child class from the virtually inherited class, and following the Trac:Coding_Conventions recommending that too,
this patch removes the virtual inheritance by contiuing to inherit all base classes (but not virtually), passing the IGUIObject& *this reference to the base classes, and registering the GetTextSize function in the leaf class (CText).
Hence the patch removes the forced workaround for the `dynamic_cast` and allows future code like D1346 to be introduced without virtual inheritance.