Model loading; overlay properties earlier

This is to permit an <overlay> section in <model> to be present whilst the rest of the model is loaded - otherwise the overlay is only present after the model is loaded;

e.g. to allow a model specific registration to be applied to a shared model;

    <model>
        <overlay>
            <texture-file>island_68.png</texture-file>
            <carrier-name>Nimitz</carrier-name>
            <carrier-registration>CVN-68</carrier-registration>
        </overlay>
        <path>nimitz-class.xml</path>
    </model>

simgear/scene/model/SGReaderWriterXML.cxx

@@ -528,7 +528,10 @@ sgLoad3DModel_internal(const SGPath& path,
     // Check for an XML wrapper
     if (modelpath.extension() == "xml") {
        try {
-            readProperties(modelpath, props);
+           if (overlay)
+               copyProperties(overlay, props);
+
+           readProperties(modelpath, props);
         } catch (const sg_exception &t) {
             SG_LOG(SG_IO, SG_DEV_ALERT, "Failed to load xml: "
                    << t.getFormattedMessage());
@@ -538,9 +541,6 @@ sgLoad3DModel_internal(const SGPath& path,
             addTooltipAnimations(path, props, model, options->getAutoTooltipsMasterMax());
         }
         
-        if (overlay)
-            copyProperties(overlay, props);
-
         if (previewMode && props->hasChild("nopreview")) {
             return std::make_tuple(0, (osg::Node *) NULL);
         }