* 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
* Steam_UserStats: use const ref when getting the stats list/map
* Steam_UserStats: don't fail in `GetStat()`/`GetAchievement()` if output buffer is null
* change datatype for default int stat, should be signed
* some logging & comments
* use const str as a param for defined_achievements_find
* GetStat()/SetStat(): return false if given name is not defined by stats.txt
* according to docs:
GetAchievement()/SetAchievement()/ClearAchievement()/GetAchievementAndUnlockTime()/IndicateAchievementProgress()
will always return true if the name was not null and exists in Admin page (achievements.json)
* ClearAchievement(): don't write user data unless json object doesn't exist, or key "earned" is true
* reduce the code in GetAchievementDisplayAttribute()
* according to docs IndicateAchievementProgress() returns false if the achievement is already unlocked
* many logging
