Check seamless cubemap extension before trying to enable it on GL2. Related to #1380.

Source/Urho3D/Graphics/OpenGL/OGLGraphics.cpp

@@ -2517,7 +2517,8 @@ void Graphics::Restore()
         }
 
         // Enable seamless cubemap if possible
-        glEnable(GL_TEXTURE_CUBE_MAP_SEAMLESS);
+        if (gl3Support || GLEW_ARB_seamless_cube_map)
+            glEnable(GL_TEXTURE_CUBE_MAP_SEAMLESS);
 #endif
 
         // Set up texture data read/write alignment. It is important that this is done before uploading any texture data