Mac: Immediately update current OpenGL context's shape.

Previously we were postponing our -[NSOpenGLContext update] call to the next
SDL_GL_SwapWindow, even if the context was current on the current thread. This
changes it so that we will do the update immediately if it's the current
context.

If you're rendering on another thread, you need to call SDL_GL_SwapWindow once
after a resize event to ensure your drawable will produce non-garbage data.

Bug: https://bugzilla.libsdl.org/show_bug.cgi?id=2339

--HG--
extra : rebase_source : 3316bbdbaf00cbbec9bcdcc5ef3dd11b814b1dc9
extra : amend_source : 15988f6bf499cd6d4ffa914988805ab298ee7661