Tweak start-captive-wmaker.sh to behave a little more nicely.

* Select $DISPLAY dynamically because X11 likes :0 and Wayland likes :1 and who
knows what else might like some other value.
* Kill Xephyr after wmaker exits.
* 640x480 should be big enough for anyone. (And the window shouldn't get in the
way so much.)

start-captive-wmaker.sh

@@ -77,10 +77,18 @@ if [ -n "$1" -a -x "$WindowMaker$1" ] ; then
     shift
 fi
 
-Xephyr -screen 1080x760 :1 &
+for i in $(seq 5 10) ; do
+    if [ "x$DISPLAY" != "x:$i" ] ; then
+        xephyr_display=":$i"
+        break
+    fi
+done
+echo "Running Xephyr on display $xephyr_display"
+
+Xephyr -screen 640x480 "$xephyr_display" &
 xephyr_pid=$!
-DISPLAY=:1 gdb \
+DISPLAY="$xephyr_display" gdb \
            --directory "$project_base" \
            --quiet \
-           --args "$WindowMaker" -display :1 --for-real "$@"
+           --args "$WindowMaker" -display "$xephyr_display" --for-real "$@"
 kill $xephyr_pid