Player's head as an icon for offline players

[helmasaur]

Feature Description: A layer has been added for offline players (#451). Althought, it would be nice to be able to display as an icon the player's head. Even better if it was in a black and white version).

• Additional context: For now, the default is offlineuser which ressembles to "👤". The closest possible thing to do is to use the same player's head for all the offline players. This is the piece of settings for this functionality: https://github.com/webbukkit/dynmap/blob/105d4f1b62539486fbaf44b354922bade041bbfc/forge-1.20/src/main/resources/configuration.txt#L129-L135