Fix bug in multires performance improvements (fixes #935).

This seems to be caused by a race condition, but adding a check to make sure
the node is defined seems to fix it.

src/js/libpannellum.js

@@ -1278,8 +1278,10 @@ function Renderer(container) {
                 }
                 var node = texturesLoading[path];
                 delete texturesLoading[path];
-                node.texture = texture;
-                node.textureLoaded = loaded ? 2 : 1;
+                if (node !== undefined) {
+                    node.texture = texture;
+                    node.textureLoaded = loaded ? 2 : 1;
+                }
             });
         };
         processNextTile = function(node) {