OnUodareFrame() and OnRenderFrame() should not be overridden.
See
http://www.opentk.com/files/doc/class_open_t_k_1_1_game_window.html#abc3e3a8c21a36d226c9d899f094152a4
Fix warnings.
Fix 3DCube invisible problem by wasted GL.End() removal and more mods.
* Separate DrawCube procedure to improve code readability.
* Move keyUp event checking from UpdateFrame to Keyboard.KeyUp.
Merge pull request #4 from fenrir-naru/visible_3dcube

Fix 3d Cube invisible problem