Fix: Incorrect thumbnail size being displayed sometimes when zooming quickly

Setting the "zoom-level" metadata asynchronously sometimes leads to the thumbnailer
loading the wrong thumbnail size. It is likely that this is caused  because the metadata operation slows down
the thumbnailing operation which, in turn, completes after the main zoom logic completes.
2 jobs for !114 with zoom in 7 minutes and 5 seconds
detached
Status Job ID Name Coverage
  Build
passed #15790
build

00:03:10

 
  Distcheck
passed #15791
distcheck

00:03:54