river: Generalize dual-screen wallpaper setup

Instead of checking for the specific DP-3 and DP-5 setup that my two
screens default to, we just check that two DP- monitors are connected
and set up the wallpapers on them.
This also circumvents the issue that screens receive different numbering
when disconnected and reconnected at any point.

desktop/.config/river/init

@@ -235,16 +235,17 @@ for pad in $(riverctl list-inputs | grep -i touchscreen); do
 done
 
 setxkbmap -option "compose:menu"
+
 # set a nice wallpaper
-if exist wlopm && exist swww; then
+if exist swww; then
     riverctl spawn "swww init"
-    if [ -e "$HOME/pictures/wall.jpg" ]; then
+    outputs=$(swww query | cut -d':' -f1)
+    if [ "$(echo "$outputs" | grep -c -e '^DP')" -eq 2 ] && [ -e "$HOME/pictures/wall_r.jpg" ]; then
+        swww img -o "$(echo "$outputs" | tail -n1)" "$HOME/pictures/wall_l.jpg"
+        swww img -o "$(echo "$outputs" | head -n1)" "$HOME/pictures/wall_r.jpg"
+    elif [ -e "$HOME/pictures/wall.jpg" ]; then
         swww img "$HOME/pictures/wall.jpg"
     fi
-    if wlopm | grep -q DP-5 && wlopm | grep -q DP-3 && [ -e "$HOME/pictures/wall_l.jpg" ] && [ -e "$HOME/pictures/wall_r.jpg" ]; then
-        swww img -o DP-5 "$HOME/pictures/wall_l.jpg"
-        swww img -o DP-3 "$HOME/pictures/wall_r.jpg"
-    fi
 fi
 
 # start status bar