Only promote windows to upper layers if one of the group members is in fullscreen mode

We want panels to be behind fullscreen windows, but osd windows above
them. With the current layer promotion logic, the panel will be promoted
to the osd layer, which is undesirable. This change restores the old
behavior by allowing layer promotion only to ActiveLayer.

BUG: 436985

src/layers.cpp

@@ -490,8 +490,8 @@ static Layer layerForClient(const X11Client *client)
             } else if (member->screen() != client->screen()) {
                 continue;
             }
-            if (member->layer() > layer) {
-                layer = member->layer();
+            if (member->layer() == ActiveLayer) {
+                return ActiveLayer;
             }
         }
     }