Require native-order direct buffers for glXXXPointer APIs.

This was always a documented restriction, but was not enforced by the runtime until now.

Until now, if you passed in some other kind of buffer, it would sometimes work, and
sometimes fail. The failures happened when the Java VM moved the buffer data while
OpenGL was still holding a pointer to it.

Now we throw an exception rather than leaving the system in a potentially bad state.