Blender Git Loki

Fix T49829: Removal of custom icon previews during add-on unregister crashes Blender.

Issue was happening when removal of custom icons was done while they
were still being rendered by preview job.

Now add a 'deffered deletion' system, to prevent main thread to delete
preview image until loading thread is done with them.

Note that ideally, calling `ED_preview_kill_jobs()` on custom icon
removal would have been simpler, but we don't have easy access to
context here...