Generate faction icon using only vanilla resources

[gar-mil]

As of v1.0, faction icons are generated dynamically using Viscious Speed's CC0 icon set as a base.

All of the other resources used in generated eras are vanilla, so this creates a singular point of unwanted behavior when a player without the add-on installed attempts to join the game of a player using a randomly generated era.

To do:

• Option to randomly generate a faction icon using only vanilla Wesnoth image files.