When building with --enable-daala_ec, calls to aom_write() and aom_read()
 use the daala entropy coder to write and read bits.
When the probability is exactly 0.5 (128), then raw bits are used.

ntt-short-1:

          MEDIUM (%) HIGH (%)
    PSNR -0.027556  -0.020114
 PSNRHVS -0.027401  -0.020169
    SSIM -0.027587  -0.020151
FASTSSIM -0.027592  -0.020102

subset1:

         RATE (%)  DSNR (dB)
    PSNR 0.03296  -0.00210
 PSNRHVS 0.03537  -0.00281
    SSIM 0.03299  -0.00161
FASTSSIM 0.03458  -0.00111

Change-Id: I48ad8eb40fc895d62d6e241ea8abc02820d573f7