Using the following sequence

SDL_Init(..:)
SDL_CreateWindow(..., SDL_WINDOW_OPENGL)
SDL_DestroyWindow
SDL_CreateWindow(..., SDL_WINDOW_OPENGL)

SDL will crash in X11_GL_GetVisual. This is due to the fact that
during SDL_DestroyWindow X11_GL_Shutdown was called because the last window
has been closed.

On the next call to SDL_CreateWindow the library is still loaded and only the
memory is reinitialized. Function pointers such as gl_data->glXChooseVisual
will not be reinitialized.

Consequently, SDL will crash due to a NULL pointer access.

The attached patch corrects the behaviour.

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%403174