The grid queues no render behind an unedited raw's quick thumbnail, since the embedded JPEG is
meant to be the thumbnail. But whether that JPEG can be read is only known when the quick job
runs, and `quick_thumb_job` had no fallback: when the preview loader returned nothing the job
failed with "no quick preview", the failure was remembered for that key, and the cell stayed
empty until an edit changed it. The quick job now carries the real thumbnail job as its last
resort, as `quick_view_job` already does for the loupe.
Marco Arnulfo
c10ca4e958