Replace get_user_wall with get_image and get_wall_list

multi-image.sh

# populate $WALL_LIST depending on number of displays and images passed
get_wall_list() {

	local paths=("$@")
	declare -ga WALL_LIST

	# multiple images and spanning conflict, bail out
	if [ "${#paths[@]}" -gt 1 ] && [ "$span_image" = true ]; then
		echo "ERROR: can't use --span with multiple images"
		exit 1
	fi

	# if # paths is 1
	if [ "${#paths[@]}" -eq 1 ]; then
		for ((i=0; i<${#DISPLAY_LIST[@]}; i++)); do
			# add same image to $WALL_LIST for each display
			get_image "${paths[0]}"
		done
    
	# if # of paths equals # of displays
	elif [ ${#paths[@]} -eq "${#DISPLAY_LIST[@]}" ]; then
		for ((i=0; i<${#DISPLAY_LIST[@]}; i++)); do
			# add each image to $WALL_LIST
			get_image "${paths[$i]}"
		done
	
	# if # of paths differ from # of display, bail out
	else
		echo "ERROR: ${#paths[@]} images provided for ${#DISPLAY_LIST[@]} displays!"
		exit 1
	fi
}

# get image path, append to $WALL_LIST
get_image() {

	local path="$1"

	# we have a file
	if [ -f "$path" ]; then
		WALL_LIST+=("$path")
		return
	# we have a directory
	elif [ -d "$path" ]; then
		dir=("$path"/*)
		rdir="${dir[RANDOM % ${#dir[@]}]}"
		get_image "$rdir" # <-- calls itself
	# not file or directory, bail out
	else
		echo "ERROR: invalid path" "$path"
		exit 1
	fi

}