Improve lookahead-threads auto selection
Smarter decision to improve fast-first-pass performance in 2-pass encodes.

Tested on a quad-core Ivy Bridge (12 threads):
Fast first pass:
veryfast:     ~7% faster
faster:      ~11% faster
fast/medium: ~15% faster
slow/slower: ~42% faster
veryslow:    ~55% faster
CRF:
veryfast:     ~9% faster
(all others remained the same)