In PbBbP pyramid structure, the last "b" cannot use temporal because L0Ref0(L1Ref0) != L0Ref0.
Don't even bother analyzing it, just use spatial.
Should improve speed and direct auto effectiveness in CRF and 1-pass modes when b-pyramid is used.
Also makes --direct temporal useful with --b-pyramid, since it will fall back to spatial for frames where temporal is broken.