encoder: fix deadlocks/crashes when picture is a single CTU row

1. if only one row per frame, disable WPP it is pointless
2. if only one row per frame, frame parallelism is (mostly) impossible
3. clamp rateControlUpdateStats() to be within row count range
cleanup: remove unused m_upBuff2
sao: move m_upBuff* to stack, fixes (#69)
Merge with stable