Due to the recent change in which scene items now only update their
transforms during the next frame's tick, snapping sources would
sometimes jitter and have incorrect snapping.  This fixes that issue by
reverting to the behavior of updating the transform immediately rather
than deferring the update to the next frame tick, but only on non-group
items.

This should not be done on items that are sub-items of groups because we
rely on the obs_scene_item::update_transform variable to update the
parent group's transform in addition.