cocoa: fix deadlock during initialization

Thanks to @wm4 for catching the bug.

Fixes #405

osdep/macosx_application.m

@@ -323,7 +323,8 @@ int cocoa_main(mpv_main_fn mpv_main, int argc, char *argv[])
         pthread_create(&playback_thread_id, NULL, playback_thread, &ctx);
 
         [mpv_shared_app().input_ready lock];
-        [mpv_shared_app().input_ready wait];
+        while (!mpv_shared_app().inputContext)
+            [mpv_shared_app().input_ready wait];
         [mpv_shared_app().input_ready unlock];
 
         cocoa_run_runloop();