game: allow overriding the config directory location (#3477)

This is primarily driven for proper mod-support. Mods would like to
isolate their settings and saves (potentially) and that is currently
done by find-and-replacing code before building. Bad!

Additionally, this has the side-effect of allowing for portable
installations of the game so, win-win.

Testing in progress, i'll merge once it is ready.

game/system/hid/display_manager.cpp

@@ -12,6 +12,7 @@ DisplayManager::DisplayManager(SDL_Window* window)
   prof().instant_event("ROOT");
   {
     auto p = scoped_prof("display_manager::init");
+    m_display_settings.load_settings();
 #ifdef _WIN32
     // Windows hint to disable OS level forced scaling and allow native resolution at non 100%
     // scales