Driver can now open whatever library is specified in SDL_GL_LoadLibrary()
 call (previously, it ignored this parameter), and uses the default system
 library when NULL is specified.

Also, library is loaded once in SDL_GL_LoadLibrary() and not every call to
 SDL_GL_GetProcAddress().

--HG--
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/trunk%401184