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...
5f0933f07a