delay subsystems, detach opengl load thread, post background color on init

src/twn_audio.c

@@ -250,6 +250,8 @@ void audio_play(const char *path,
                 float panning)
 {
     if (!ctx.audio_initialized) {
+        SDL_InitSubSystem(SDL_INIT_AUDIO);
+
         profile_start("audio initialization");
 
         SDL_AudioSpec request, got;