[osg-submissions] Fix "hidden virtual" warnings introduced since 2.2.0

[Andrew Bettison]

Robert is right, the "hidden virtual" warnings I want to eliminate 
indicate neither a bug in the compiler nor in OSG, because in this 
particular case, either (a) the methods in question are never invoked 
via a pointer to the subclass, only via a pointer to the base class (eg, 
osg::NodeVisitor), in which case they are not hidden and the 
polymorphism works exactly as intended, or (b) the C++ argument 
overloading semantics result in the correct (intended) method of the 
subclass being called (eg, in the case of osg:NodeVisitor, the 
inheritance graph of osg::Node and its subclasses exactly matches all 
the defined osg::NodeVisitor::apply() base methods).

In any case, warnings from OSG headers are still a problem for me, and 
probably will be for others in future, too, since warnings-as-errors is 
a common practice, and it's inadvisable to suppress this particular 
warning, since in the general case, an unintentionally hidden virtual 
method can indeed be a bug and a hard to spot one at that.

So I am re-submitting my patch, this time replacing all the extra 
methods with "using" declarations (as Tim Moore suggested), one per 
subclass wherever an overloaded virtual is overridden.  This does not 
introduce any new code that could make OSG more fragile, since the 
additions are not "code" per se (they are declarations).  I hope that 
this patch will pass muster where my original failed.  My only concern 
is that "using" declarations might not be supported by all C++ compilers 
currently in use by OSG users.  However, the "using" declaration has 
been part of the OSI C++ standard since 1996.

-- Andrew Bettison

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