Mac: Run the CFRunLoop in joystick mode during SDL_SYS_JoystickNeedsPolling().

This fixes hotplugging failing to detect devices.

src/joystick/darwin/SDL_sysjoystick.c

@@ -604,7 +604,9 @@ SDL_SYS_JoystickDetect()
 SDL_bool
 SDL_SYS_JoystickNeedsPolling()
 {
-	// BUGBUG - only works if someone else is pumping the CFRunLoop...
+    while (CFRunLoopRunInMode(SDL_JOYSTICK_RUNLOOP_MODE,0,TRUE) == kCFRunLoopRunHandledSource) {
+        /* no-op. Pending callbacks will fire in CFRunLoopRunInMode(). */
+    }
     return s_bDeviceAdded || s_bDeviceRemoved;
 }