nal: VPS startCodeprefix needs 4 bytes

Issue pointed out by Zakk Saito, Pegasys
sei: m_lastBPSEI is overwritten each time

Issue reported by Zakk Saito, Pegasys
refine intra neighbors
Added cmake support to pass along build flags to yasm.

This is particularly helpful when debugging.
asm & testbench: psyCost_pp_4x4 in sse4: improve 2088c->337c