ViewPhoto: take filter into account for prev/next links

controllers/ViewPhotoAlbum.php

@@ -94,8 +94,15 @@ class ViewPhotoAlbum extends HTMLController
 		{
 			$index = new PhotosIndex($mosaic, Registry::get('user')->isAdmin());
 			$this->page->adopt($index);
-			if ($id_tag > 1)
-				$index->setUrlSuffix('?in=' . $id_tag);
+
+			$url_params = [];
+			if (isset($tag) && $tag->id_parent != 0)
+				$url_params['in'] = $tag->id_tag;
+			if (!empty($active_filter))
+				$url_params['by'] = $active_filter;
+
+			if (!empty($url_params))
+				$index->setUrlSuffix('?' . http_build_query($url_params));
 		}
 
 		// Make a page index as needed, while we're at it.