This was happening late enough during main() that there were already
non-exclusive threadpools/threads created with regular scheduler
settings. Those settings would be cached in recent glib, creating
disparities that it will g_error() out on later. Those created threads
might however be reused later on by different code (eg. metadata
extraction, directly or indirectly), with the regular scheduling
priorities set.

Given that even accessing GSettings will result in threads being
spawned underneath, there's no better choice than doing this always.
This means the 'sched-idle' setting is ineffective. But this default
should avoid fingers from pointing at us.