GL_RGB5 is probably more correct as GL_UNSIGNED_SHORT_5_6_5

* It has one bit more precision in green, but it has the right number of
  channels, so it's a better fit than GL_UNSIGNED_SHORT_5_5_5_1
Try to avoid hooking into module that was unloaded elsewhere.

* There is still a small race condition, but worst case if it's hit then
  we load a library and free it, and the only way to protect it is to lock
  around FreeLibrary which is quite scary.