Add VVV_exit()

This is just a function that calls the cleanup() in main.cpp, as well as
calls exit().

I would have liked to use SDL_ExitProcess() here, because that function
has ifdefs for different runtime environments. But alas, it's an
internal function and isn't exported. Ah well; exit() seems to be fine
anyway.

desktop_version/src/main.cpp

@@ -377,6 +377,12 @@ static void cleanup()
     FILESYSTEM_deinit();
 }
 
+void VVV_exit(const int exit_code)
+{
+    cleanup();
+    exit(exit_code);
+}
+
 static void inline deltaloop()
 {
     //timestep limit to 30