Mac: Remove dead FULLSCREEN_TOGGLEABLE code.

This code was written almost 2 years ago, and the flag hasn't been
changed since. Cleaning up the code by removing the conditional blocks,
so that they behave the same way they have for the past two years.

FULLSCREEN_TOGGLEABLE used to cause us to use
-[NSOpenGLContext setFullScreen] and a pixel format with
NSOpenGLPFAFullScreen.
SDL_GL_MakeCurrent: Only no-op redundant calls on *same* thread.

This ensures that we only no-op redundant calls if they're made on the
same thread as the last MakeCurrent with those arguments. This should
maek SDL behave better with multithreaded environments.