[client] fix shutdown race condition with the frame thread

client/src/main.h

@@ -49,6 +49,8 @@ struct AppState
   void               * lgrData;
   bool                 lgrResize;
 
+  SDL_Thread         * t_frame;
+
   const LG_Clipboard * lgc;
   SpiceDataType        cbType;
   struct ll          * cbRequestList;
@@ -130,4 +132,4 @@ struct KeybindHandle
 
 // forwards
 extern struct AppState  state;
-extern struct AppParams params;
+extern struct AppParams params;