Mac OS X: Look for joystick hotplug in its own CFRunLoop.

This allows the joystick hotplug to function without the main event loop
 (specifically: without SDL_INIT_VIDEO), and moves explicit polling for
 joysticks where it belongs at the low-level: in SDL_SYS_JoystickDetect().

This lets apps call SDL_JoystickUpdate() to get hotplug events and keep
 SDL_NumJoysticks() correct, as expected. As SDL_PumpEvents() (and
 SDL_PollEvents, etc) calls SDL_JoystickUpdate(), existing apps will function
 as before.

Thanks to "raskie" on the forums for pointing this out!

--HG--
extra : rebase_source : 09d8e949731014ba3226aecf73050e35a54a5cdd