PRs #147511 and #147595 ensured that any check for scene strip renders
in the prefetch thread would also recurse into "sequencer-type" scene
strips, but this introduced the potential for an infinite recursion.
Fix by using `SeqRenderState` to detect such a recursion.
Also remove unused `seqbasep` context member in `PrefetchJob`, and
add a check for non-nullptr `Editing`, fixing a crash where the scene was
present without an `Editing` counterpart.
Pull Request: https://projects.blender.org/blender/blender/pulls/147949
a8e64f9a22