Include OpenGL headers instead of trying to be smart.

I hoped the typedef was the same everywhere, but apparently ES has
something else, which is typedef'd recursively and I don't want to
maintain the consequences when porting elsewhere.

src/MeshView.h

@@ -29,12 +29,9 @@
  */
 
 #include "Magnum.h"
+#include "OpenGL.h"
 #include "magnumVisibility.h"
 
-#ifndef DOXYGEN_GENERATING_OUTPUT
-typedef std::ptrdiff_t GLintptr;
-#endif
-
 namespace Magnum {
 
 /**