[herbstluftwm] Update ‘random-wallpaper’ to use screen size

Instead of assuming that the screen is 3840x1080, check the size of the ‘root’
window.

herbstluftwm/.config/herbstluftwm/autostart

@@ -11,8 +11,12 @@ function hc() { herbstclient "$@"; }
 
 # This function picks a random wallpaper from a list collection.
 function random-wallpaper() {
-    find ~/pictures/wallpapers -type f \
-        -wholename '*/3840x1080/*' -o -wholename '*/patterns/*' \
+    local rootinfo=$(xwininfo -root)
+    local width=$(echo $rootinfo | grep 'Width:' | sed 's/.*Width: \([[:digit:]]\+\).*/\1/')
+    local height=$(echo $rootinfo | grep 'Height:' | sed 's/.*Height: \([[:digit:]]\+\).*/\1/')
+
+    find ~/pictures/wallpaper -type f \
+        -wholename "*/${width}x${height}/*" -o -wholename '*/patterns/*' \
         | shuf -n 1
 }