Backport swap() -> V_swap() rename to avoid C++11 ADL ambiguity errors

The swap() function provided in the MathLib header was renamed to V_swap in recent Source SDK versions (e.g. 2013) to avoid causing ambiguity problems with std::swap(). But older SDK versions (such as TF2) lack this change, as they predate it.

The ambiguity between MathLib's swap() and std::swap() causes considerable problems when using newer features of C++ (such as std::unique_ptr) which internally call swap() in an unqualified manner to implement move semantics:

/usr/include/c++/5.2.0/bits/unique_ptr.h:342:6: error: call of overloaded ‘swap(MyType*&, MyType*&)’ is ambiguous
/usr/include/c++/5.2.0/bits/move.h:176:5: note: candidate: void std::swap(_Tp&, _Tp&) [with _Tp = MyType*]
hl2sdk-tf2/public/mathlib/mathlib.h:611:18: note: candidate: void swap(T&, T&) [with T = MyType*]

This patch backports the swap() -> V_swap() rename from the 2013 SDK version to the TF2 SDK version, so that the TF2 SDK can be used in conjunction with C++11 features such as std::unique_ptr without difficulty.

More information on why swap() isn't called in a namespace-qualified manner by standard library functions:
http://en.cppreference.com/w/cpp/language/adl#Notes

game/client/clientleafsystem.cpp

@@ -1722,8 +1722,8 @@ void CClientLeafSystem::SortEntities( const Vector &vecRenderOrigin, const Vecto
 		{
 			if( dists[i] > dists[i+stepSize] )
 			{
-				swap( pEntities[i], pEntities[i+stepSize] );
-				swap( dists[i], dists[i+stepSize] );
+				::V_swap( pEntities[i], pEntities[i+stepSize] );
+				::V_swap( dists[i], dists[i+stepSize] );
 
 				if( i == 0 )
 				{