Assets: Generate light preview when making light object an asset

We already supported previews for lights, just didn't actually use them
when making a light object an asset. They were only used when making the
light data itself an asset.

source/blender/editors/interface/interface_icons.c

@@ -2041,6 +2041,14 @@ void UI_icon_render_id(
     return;
   }
 
+  /* For objects, first try if a preview can created via the object data. */
+  if (GS(id->name) == ID_OB) {
+    Object *ob = (Object *)id;
+    if (ED_preview_id_is_supported(ob->data)) {
+      id = ob->data;
+    }
+  }
+
   if (!ED_preview_id_is_supported(id)) {
     return;
   }