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

@@ -467,9 +467,16 @@ void VertexDeclaration::activateShaderPipeline(GLSL::Program* _prog) const
       glVertexAttribPointer(loc, pElem->numElements_,  pElem->type_, normalizeElem, vertexStride, pElem->pointer_);
 
       if (supportsInstancedArrays())
+      {
+        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;
 
@@ -499,9 +506,16 @@ void VertexDeclaration::deactivateShaderPipeline( GLSL::Program* _prog ) const
       glDisableVertexAttribArray(loc);
 
       if (supportsInstancedArrays() && pElem->divisor_)
+      {
+        if( openGLVersion(3,3) )
         {
           glVertexAttribDivisor(loc, 0);
         }
+        else
+        {
+          glVertexAttribDivisorARB(loc, 0);
+        }
+      }
     }
   }
 }