Allow more frame parallelism in LP-gop
Allow more frame parallelism in LP-gop

Add dependency to the reference frame instead of the previous frame,
in order to allow more frames to be encoded in parallel when temporal
stepping >1 in LP-gop (such as --gop=lp-g8d4r1t2).

ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar