refactor: Enable unreachable cleanup logic in main()

> refactor `main_implemntation()` cleanup to run once on `launcher::shutdown()`, not sure why it was defined like that (unreachable code, at least for iidx)
> not thread safe, but probably not needed (no obvious failures as well)
> related to #1

launcher/launcher.cpp

@@ -2067,9 +2067,8 @@ int main_implementation(int argc, char *argv[]) {
         game->post_attach();
     }
 
-    // game start
-    log_info("launcher", "calling game entry");
-    avs::game::entry_main();
+    // cleanup procedure
+    launcher::on_shutdown = [&]() {
 
     // clear presence
     richpresence::shutdown();
@@ -2158,6 +2157,11 @@ int main_implementation(int argc, char *argv[]) {
 #ifdef SPICE64
     games::iidx::camera_release();
 #endif
+    };
+
+    // game start
+    log_info("launcher", "calling game entry");
+    avs::game::entry_main();
 
     // shutdown
     log_warning("launcher", "end");

launcher/shutdown.cpp

@@ -15,11 +15,15 @@
 
 namespace launcher {
 
+    std::function<void()> on_shutdown = []() {};
+
     void stop_subsystems() {
         // note that it is possible for stop_subsystems to be called multiple times
         // (e.g., crashing, and then closing the window)
         // therefore, subsystems need to be guarded against multiple unload attempts
         log_info("launcher", "stopping subsystems");
+        on_shutdown(); //_BUG: not thread safe, required?
+        on_shutdown = []() {};
 
         // flush/stop logger
         logger::stop();

launcher/shutdown.h

@@ -2,8 +2,10 @@
 
 #include <windows.h>
 #include <stdlib.h>
+#include <functional>
 
 namespace launcher {
+    extern std::function<void()> on_shutdown;
     void stop_subsystems();
     void kill(UINT exit_code = EXIT_FAILURE);
     void shutdown(UINT exit_code = EXIT_SUCCESS);