With undo/redo after creating assets, previews of the "Current File"
asset library would often go missing, e.g. see T83884.
What I thought was just a simple error with flags, turned out to be a
bigger problem:
The preview generation in a background job will eventually modify ID
data, but the undo push was done prior to that. So obviously, an undo
then would mean the preview is lost.
This patch makes it so undo/redo will regenerate the preview, if the preview
rendering was invoked but not finished in the undone/redone state.
The preview flag PRV_UNFINISHED wasn't entirely what we needed. So I had to
change it to a slightly different flag.