Load external libraries only once.

source/external/externallibs.js

@@ -1,15 +1,26 @@
 OV.ExternalLibLocation = null;
 
+OV.LoadedExternalLibs = {};
+
 OV.LoadExternalLibrary = function (libName, callbacks)
 {
     if (OV.ExternalLibLocation === null) {
         callbacks.error ();
         return;
     }
+
+    if (OV.LoadedExternalLibs[libName] !== undefined) {
+        callbacks.success ();
+        return;
+    }
+
     let scriptElement = document.createElement ('script');
     scriptElement.type = 'text/javascript';
     scriptElement.src = OV.ExternalLibLocation + '/' + libName;
-    scriptElement.onload = callbacks.success;
+    scriptElement.onload = function () {
+        callbacks.success ();
+        OV.LoadedExternalLibs[libName] = true;
+    };
     scriptElement.onerror = callbacks.error;
     document.head.appendChild (scriptElement);
 };

source/external/rhinoexporter.js

@@ -15,10 +15,17 @@ OV.Exporter3dm = class extends OV.ExporterBase
 	{
 		if (this.rhino === null) {
 			let obj = this;
-			rhino3dm ().then (function (rhino) {
-				obj.rhino = rhino;
-				obj.ExportRhinoContent (model, files, onFinish);
-			});
+			OV.LoadExternalLibrary ('rhino3dm.min.js', {
+				success : function () {
+                    rhino3dm ().then (function (rhino) {
+                        obj.rhino = rhino;
+                        obj.ExportRhinoContent (model, files, onFinish);
+                    });
+                },
+				error : function () {
+					onFinish ();
+				}
+			});            
 		} else {
 			this.ExportRhinoContent (model, files, onFinish);
 		}