libvpx

Adaptive mode search scheduling

This commit enables an adaptive mode search order scheduling scheme
in the rate-distortion optimization. It changes the compression
performance by -0.433% and -0.420% for derf and stdhd respectively.
It provides speed improvement for speed 3:

bus CIF 1000 kbps
24590 b/f, 35.513 dB, 7864 ms ->
24696 b/f, 35.491 dB, 7408 ms (6% speed-up)

stockholm 720p 1000 kbps
8983 b/f, 35.078 dB, 65698 ms ->
8962 b/f, 35.054 dB, 60298 ms (8%)

old_town_cross 720p 1000 kbps
11804 b/f, 35.666 dB, 62492 ms ->
11778 b/f, 35.609 dB, 56040 ms (10%)

blue_sky 1080p 1500 kbps
57173 b/f, 36.179 dB, 77879 ms ->
57199 b/f, 36.131 dB, 69821 ms (10%)

pedestrian_area 1080p 2000 kbps
74241 b/f, 41.105 dB, 144031 ms ->
74271 b/f, 41.091 dB, 133614 ms (8%)

Change-Id: Iaad28cbc99399030fc5f9951eb5aa7fa633f320e