The `!BKE_previewimg_is_finished()` in `icon_preview_startjob_all_sizes()`
would fail.
Caused by 990bd9acf243ae. We have to be more picky about tagging previews as
unfinished after file read. But we also have to consider files stored in old
versions and set the unfinished tag as needed.
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.
* Support defining (not necessarily rendering) icons in threads. Needed so the
File Browser can expose file previews with an icon-id to scripts.
** For that, ported `icons.c` to C++, to be able to use scope based mutex locks
(cleaner & safer code). Had to do some cleanups and minor refactoring for
that.
* Added support for ImBuf icons, as a decent way for icons to hold the file
preview buffers.
* Tag previews as "unfinished" while they render in a thread, for the File
Browser to dynamically load previews as they get finished.
* Better handle cases where threaded preview generation is requested, but the
ID type doesn't support it (fallback to single threaded). This is for general
sanity of the code (as in, safety and cleanness)
* Enabled asset notifier for custom preview loading operator, was just disabled
because `NC_ASSET` wasn't defined in master yet.
Part of the first Asset Browser milestone. Check the #asset_browser_milestone_1
project milestone on developer.blender.org.
Differential Revision: https://developer.blender.org/D9719
Reviewed by: Bastien Montagne, Brecht Van Lommel
