WaveFront: add new function to enable all rows
Lookahead: implement wavefront parallel processing
intra: move intra_pred_dc to intra-sse41.cpp; it uses SSSE3 instructions

We don't have an intra-ssse3.cpp and it seems a waste to create one just for
this one function.
remove reduce register copy in FILTER_H4_w2_2 and FILTER_H4_w4_2