fixed segfault, when OpenGL Version less than 3.3 is used because

glVertexAttribDivisor is a nullpointer. Use ARB extension call instead

libs_required/ACG/GL/VertexDeclaration.cc

@@ -468,7 +468,14 @@ void VertexDeclaration::activateShaderPipeline(GLSL::Program* _prog) const
 
       if (supportsInstancedArrays())
       {
-        glVertexAttribDivisor(loc, pElem->divisor_);
+        if( openGLVersion(3,3) )
+        {
+          glVertexAttribDivisor(loc, pElem->divisor_);
+        }
+        else
+        {
+          glVertexAttribDivisorARB(loc, pElem->divisor_);
+        }
       }
       else if (pElem->divisor_)
         std::cerr << "error: VertexDeclaration::activateShaderPipeline - instanced arrays not supported by gpu!" << std::endl;
@@ -500,7 +507,14 @@ void VertexDeclaration::deactivateShaderPipeline( GLSL::Program* _prog ) const
 
       if (supportsInstancedArrays() && pElem->divisor_)
       {
-        glVertexAttribDivisor(loc, 0);
+        if( openGLVersion(3,3) )
+        {
+          glVertexAttribDivisor(loc, 0);
+        }
+        else
+        {
+          glVertexAttribDivisorARB(loc, 0);
+        }
       }
     }
   }