introduce audio_model = "push" to twn.toml

src/twn_engine_context_c.h

@@ -83,6 +83,7 @@ typedef struct EngineContext {
     /* signals mouse focus, used to disable mouse capture */
     bool window_mouse_resident;
     bool audio_initialized;
+    bool push_audio_model;
     bool cull_faces;
 } EngineContext;