Blender Git Loki

Fix asset previews not showing in "Current File" repository after reading

Previews would be flagged as unfinished when reading. Instead clear the flag
and always consider them finished upon reading. If we wouldn't do this, the
File Browser would keep running the preview updating to wait for the preview to
finish. It could be smarter and also check if there's actually a preview job
running.
Instead, I think it will just display an unfilled buffer for now. Up until
today we wouldn't even know if a stored preview is finished or not, so it would
always assume they are finished. We do the same now, but have the option to
make it smarter.