ViewPhoto: take filter into account for prev/next links

controllers/ViewPhoto.php

@@ -46,6 +46,19 @@ class ViewPhoto extends HTMLController
 		if (isset($tag))
 			$page->setTag($tag);
 
+		// Keeping tabs on a filter?
+		if (isset($_GET['by']))
+		{
+			// Let's first verify that the filter is valid
+			$contributors = array_filter($tag->getContributorList(), fn($el) => $el['slug'] === $_GET['by']);
+			if (count($contributors) !== 1)
+				throw new UnexpectedValueException('Invalid filter for this album or tag.');
+
+			// Alright, let's run with it then
+			$filter = reset($contributors);
+			$page->setActiveFilter($filter['slug']);
+		}
+
 		$this->page->adopt($page);
 		$this->page->setCanonicalUrl($this->photo->getPageUrl());
 	}