lookahead primitives: fix bug that caused wrong cost estimates in ducks_take_off.
no-rdo: implemented topskip algorithm

It is basically setting the starting depth from which the partitioning should
happen for particular CU. The starting depth for particular CU in present frame
is selected form the previous frame's colocated CU minimum depth.

the performance, bitrate increase and psnr comparison are given below
CLI: x265.exe input.y4m -o abc.hevc -r recon.y4m --rd 1 --ref 1
a. Early exit OFF and Top Skip OFF
b. Early exit OFF and Top Skip ON

BasketballDrive_1920x1080_50
Timetaken to encode: 681/639s
bitrate: 3650/3657 kb/s
PSNR: 36.7/36.703
perf improvement: 6.16% (compared to TopSkip OFF and ON)
Bitrate increase: 0.19% (compared to TopSkip OFF and ON)

Cactus_1920x1080_50
Timetaken to encode: 530/492s
bitrate: 2787/2795 kb/s
PSNR: 35.527/35.529
perf improvement: 7.16% (compared to TopSkip OFF and ON)
Bitrate increase: 0.28% (compared to TopSkip OFF and ON)

Kimono1_1920x1080_24
Timetaken to encode: 278/264s
bitrate: 1243/1246 kb/s
PSNR: 38.16/38.16
perf improvement: 5.03% (compared to TopSkip OFF and ON)
Bitrate increase: 0.24% (compared to TopSkip OFF and ON)

FourPeople_1280x720_60
Timetaken to encode: 173/163s
bitrate: 486/492 kb/s
PSNR: 39.097/39.094
perf improvement: 5.78% (compared to TopSkip OFF and ON)
Bitrate increase: 1.2% (compared to TopSkip OFF and ON)

PartyScene_832x480_50
Timetaken to encode: 134/119s
bitrate: 1652/1661 kb/s
PSNR: 31.374/31.377
perf improvement: 11.16% (compared to TopSkip OFF and ON)
Bitrate increase: 0.544% (compared to TopSkip OFF and ON)

big_buck_bunny_360p24
Timetaken to encode: 1772/1477s
bitrate: 174/175 kb/s
PSNR: 37.798/37.797
perf improvement: 16.6% (compared to TopSkip OFF and ON)
Bitrate increase: 0.5% (compared to TopSkip OFF and ON)
Merge
crf: bug fix. regulate qp of first frame based on ABR_INIT_QP.
asm: remove assignements to square block sa8d[] methods

These are handled specially in x265_setup_primitives()