wallpaper: use a folder of wallpapers for multiple screens

src/.local/bin/wallpaper.example

@@ -2,4 +2,15 @@
 
 # Per host wallpaper configuration example
 
-xwallpaper --output DP-2-1 --center med/pix/dow/ml.png --output DP-1 --center med/pix/dow/mr.png --output eDP-1 --center med/pix/dow/m.png
+PREFIX="$HOME/med/wall"
+if [ -z "$1" ]; then
+	WALL="coastline_black"
+else
+	WALL="$1"
+fi
+
+FOLDER="$PREFIX"/"$WALL"
+
+xwallpaper --output DP2-1 --center "$FOLDER"/l.jpg \
+	--output DP1 --center "$FOLDER"/r.jpg \
+	--output eDP1 --center "$FOLDER"/s.jpg