define XAUTHORITY if it is unset

the mission requiring xeyes might not work if the XAUTHORITY variable is
unset. In that case, it defaults to ~/.Xauthority, which will fail
because GameShell redefines $HOME.

Also, if it is defined but starts with ~, I need to change the ~ to
$REAL_HOME.

missions/intermediate/04_bg_xeyes/gshrc

@@ -0,0 +1,9 @@
+# if the XAUTHORITY variable isn't defined, it defaults to ~/.Xauthority
+# since GameShell redefines HOME, the file won't be found.
+
+if [ -z "$XAUTHORITY" ]
+then
+  export XAUTHORITY=$REAL_HOME/.Xauthority
+else
+  export XAUTHORITY=${XAUTHORITY/~/$REAL_HOME}
+fi