Remove a unused sub-pixel search

The original iterative search was replaced by subpel_tree search,
and was not used anymore.

Change-Id: I998b38e1cb0ee359a08b2410d0766dbf183ab071
Adaptive motion control on ref and search range

This commit takes a preliminary attempt to refine the motion search
control. It detects the SAD associated with mv predictor per reference
frame, and based on which to determine whether the encoder wants to
reduce the motion search range (if the predicted mv provides fairly
small SAD), or to skip the current reference frame (if there exists
another ref frame that gives much smaller SAD cost).

This feature is turned on in the settings of speed 1 and above.

In speed 1, compression performance changed
derf  -0.018%
yt    -0.043%
hd    -0.045%
stdhd -0.281%

speed-up
pedestrian_area_1080p at 4000 kbps 100 frames
199651ms -> 188846ms (5.5% speed-up)
blue_sky_1080p at 6000 kbps
443531ms -> 415239ms (6.3% speed-up)

In speed 2, compression performance changed
derf  -0.026%
yt    -0.090%
hd    -0.055%
stdhd -0.210%

speed-up
pedstrian 113949ms -> 108855ms (4.5% speed-up)
blue_sky  271057ms -> 257322ms (5% speed-up)

Change-Id: I1b74ea28278c94fea329d971d706d573983d810d
Removing vpx_codec_vp9x_cx and internal experimental flag.

vpx_codec_vp9x_cx is not used internally. Experimental flag from
vp9_extracfg is also not really used. YUV 4:4:4 just works after these
changes (you have to specify --profile=1 for the encoder).

Change-Id: Ib1c8461d0d19d159827e005efe868f891eea0140
Merge "Remove a unused sub-pixel search"
Merge "Removing vpx_codec_vp9x_cx and internal experimental flag."
Merge "Adaptive motion control on ref and search range"