cameo/zstack.pl: output one point at (x, y, z0) to set reference level

Otherwise, gp2rml will eliminate any offset. (x, y) is an arbitrary point
from the toolpaths.

cameo/zstack.pl

@@ -54,7 +54,11 @@ while (<>) {
 		$z{$z} .= "$s\n";
 		undef $s;
 	}
-	$z = $3 if /^-?\d+(\.\d*)?\s+-?\d+(\.\d*)?\s+(-?\d+(\.\d*)?)/;
+	if (/^-?\d+(\.\d*)?\s+-?\d+(\.\d*)?\s+(-?\d+(\.\d*)?)/) {
+		$x = $1 unless defined $x;
+		$y = $2 unless defined $y;
+		$z = $3;
+	}
 	$s .= $_;
 	$zmax = $z if $z > $zmax || !defined $zmax;
 }
@@ -68,6 +72,8 @@ if ($reverse) {
 	%z = %t;
 }
 
+print "$x $y $z0\n\n";
+
 $z = $z0;
 for $c (sort { $b <=> $a } keys %z) {
 	while ($z > $c+$zs) {