encoder: disable lookahead for all-intra encodes

This works around a bug in the current Lookahead::slicetypeDecide().  I have
a patch in the works which fixes this properly, but I think this is a reasonable
workaround.  Tested with ABR and it works ok.

--HG--
branch : stable
fix hash mistake from --sao-lcu-opt=0 --frame-threads 2

Delay all deblock and SAO until the entire frame is available.  Frame SAO and
frame parallelism will now generate correct results, but it will have very poor
performance, so the hard check is relaxed to a warning.

--HG--
branch : stable