set the (default) LayoutStrategy only if it has not been set already

git-svn-id: https://svn.apache.org/repos/asf/xmlgraphics/fop/trunk@196919 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/fop/apps/Driver.java

@@ -581,7 +581,9 @@ public class Driver implements LogEnabled {
         /** LayoutStrategy is hard-wired for now, but needs to be made
         accessible through the API and/or configuration */
         if (foInputHandler instanceof FOTreeHandler) {
-            currentDocument.setLayoutStrategy(new LayoutManagerLS(currentDocument));
+            if (currentDocument.getLayoutStrategy() == null) {
+                currentDocument.setLayoutStrategy(new LayoutManagerLS(currentDocument));
+            }
         }
         treeBuilder.foTreeControl = currentDocument;
         try {