move opengl library loading to a thread that starts as soon as possible and is awaited as late as we can allow

src/twn_engine_context_c.h

@@ -28,6 +28,7 @@ typedef struct EngineContext {
     char **argv;
     /* where the app was run from, used as the root for packs */
     char *base_dir;
+    char *title;
 
     Vec2 window_dims;
     Rect viewport_rect;