* new option `upload_achievements_icons_to_gpu` in `configs.main.ini` which controls whether the overlay should upload the achievements icons to the GPU and display them or not
* synchronize overlay proc with the periodic steam callback in a better way to avoid FPS drop
* prevent overlay flickering regression
* upload achievements icons to the GPU in the overlay proc periodically, this dramatically decreased the startup locking/halt time
* fix a potential deadlock scenario in the overlay as a result of synchonizing with 2 mutex objects
