Use MGLGetProcAddress to access OpenGL functions

examples/spinning_cube/spinning_cube.h

Index: examples/spinning_cube/spinning_cube.h
diff --git a/examples/spinning_cube/spinning_cube.h b/examples/spinning_cube/spinning_cube.h
index 03529701d4609c4099f2b2a9b6e2d6432b5ac81a..500a3089b5a8ec4c2a1c7e231d98fe3654923c13 100644
--- a/examples/spinning_cube/spinning_cube.h
+++ b/examples/spinning_cube/spinning_cube.h
@@ -13,6 +13,8 @@ namespace examples {
 
 class SpinningCube {
  public:
+  class GLInterface;
+
   SpinningCube();
   ~SpinningCube();
 
@@ -42,6 +44,7 @@ class SpinningCube {
   bool initialized_;
   uint32_t width_;
   uint32_t height_;
+  scoped_ptr<GLInterface> gl_;
   scoped_ptr<GLState> state_;
   float fling_multiplier_;
   int direction_;