gobj: Fixed wrong texture pool key for 2D texture arrays

        - 2D texture arrays are added to internal storage with
        specific unique name, so keep it as texture pool key
        instead of filename.

panda/src/gobj/texturePool.cxx

@@ -772,7 +772,7 @@ ns_load_cube_map(const Filename &filename_pattern, bool read_mipmaps,
   nassertr(tex != (Texture *)NULL, NULL);
   tex->set_filename(filename_pattern);
   tex->set_fullpath(filename);
-  tex->_texture_pool_key = filename;
+  tex->_texture_pool_key = unique_filename;
 
   {
     MutexHolder holder(_lock);