Intoduced a new setGLExtensions template method to do a copy of void* pointer to

local function pointer to avoid compiler warnings related to case void*.

Moved various OSG classes across to using setGLExtensions instead of getGLExtensions,
and changed them to use typedef declarations in the headers rather than casts in
the .cpp.

Updated wrappers

src/osg/GraphicsThread.cpp

@@ -65,7 +65,6 @@ void BarrierOperation::operator () (Object* object)
 {
     if (_preBlockOp!=NO_OPERATION)
     {
-        GraphicsContext* context = dynamic_cast<GraphicsContext*>(object);
         if (_preBlockOp==GL_FLUSH) glFlush();
         else if (_preBlockOp==GL_FINISH) glFinish();
     }