Achievement / GroupAchievement metadata
Gets the icon of the achievement, which is a handle to be used in ISteamUtils::GetImageRGBA(), or 0 if none set.
A return value of 0 may indicate we are still fetching data, and you can wait for the UserAchievementIconFetched_t callback
which will notify you when the bits are ready. If the callback still returns zero, then there is no image set for the
specified achievement.