Disable XML validation and external DTD loading in TMXMapReader

This is because of a weird error, that only happens, when the jar is built manually somehow...

hacked-libtiled/tiled/io/TMXMapReader.java

@@ -821,6 +821,8 @@ public class TMXMapReader
             factory.setIgnoringComments(true);
             factory.setIgnoringElementContentWhitespace(true);
             factory.setExpandEntityReferences(false);
+            factory.setValidating(false);
+            factory.setFeature("http://apache.org/xml/features/nonvalidating/load-external-dtd", false);
             DocumentBuilder builder = factory.newDocumentBuilder();
             builder.setEntityResolver(entityResolver);
             InputSource insrc = new InputSource(in);