Thumbnails should have the mtime of the original item
We should store the modification time of the original item as tEXt::Thumb::MTime in the thumbnail PNG. It is mandated by the Thumbnail Management Standard, and even though we might have our own thumbnailing infrastructure, it will be required to update the thumbnail when the corresponding original changes.
Note that there's no need to bump THUMBNAIL_GENERATION until (a) we add code to update stale thumbnails (b) a few more important fixes add up because, unlike other kinds of content, images usually don't tend to change.