Blender Git Loki

Asset System: Various changes to previews in preparation for Asset Browser

* 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