This CL adds the use of ALTREF2_FRAME to both single / comp reference
prediction at the encoder side. In particular, the encoder keeps the
distant altref as ALTREF, and uses the internal extra altrefs to
refresh ALTREF2.

Compared with the baseline (ext_tx and global_motion disabled simply
for speed concern):
(a) lowres: avg_psnr -0.395% ovr_psnr -0.393% ssim -0.329%
(b) midres: avg_psnr -0.419% ovr_psnr -0.431% ssim -0.444%
(c) AWCY High Latency:
   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-0.6661 | -0.5988 | -0.6669 |  -0.6993 | -0.6988 | -0.7303 | -0.6051
(d) AWCY Low Latency:
  PSNR | PSNR Cb | PSNR Cr | PSNR HVS |   SSIM | MS SSIM | CIEDE 2000
0.0720 | -0.0505 |  0.1501 |   0.0670 | 0.0842 |  0.0517 |     0.0158

TODO list:
(1) To have altref2 incorporated with ext-comp-refs;
(2) To have altref2 fully work with new-multisymbol;
(3) To re-collect the initial default probs/cdfs;
(4) To tune the encoder gf group structure design for altref2.

Change-Id: I6ad63fd65afa903d3bba20acdb68e3b67acf7fdf