Don't invoke a graphics toolkit when setting the album cover from a

clean thumbnail; we can just copy it over.  Should be a decent perf
improvement in many cases.  Fixes ticket #1255.

modules/gallery/helpers/item.php

@@ -105,9 +105,15 @@ static function make_album_cover($item) {
 
     model_cache::clear();
     $parent->album_cover_item_id = $item->is_album() ? $item->album_cover_item_id : $item->id;
-    $parent->thumb_dirty = 1;
+    if ($item->thumb_dirty) {
+      $parent->thumb_dirty = 1;
+      graphics::generate($parent);
+    } else {
+      copy($item->thumb_path(), $parent->thumb_path());
+      $parent->thumb_width = $item->thumb_width;
+      $parent->thumb_height = $item->thumb_height;
+    }
     $parent->save();
-    graphics::generate($parent);
     $grand_parent = $parent->parent();
     if ($grand_parent && access::can("edit", $grand_parent) &&
         $grand_parent->album_cover_item_id == null)  {