Updating intra prediction to use adjacent TF'd 4x4 blocks.

All block sizes now using intra prediction based on just the adjacent
 neighboring 4x4 blocks from TF.  This has several benefits:

  (1) reduces complexity - no need for extra logic to TF up or TF down each
      block's neighbors when doing prediction
  (2) reduces memory footprint - only a single row's worth (plus 1) of TF'd
      4x4 blocks need to be held in memory while encoding / decoding
  (3) better prediction - training 16x16 predictors is now tractable as it
      uses only (4+1+4+4)*(4*4)=208 inputs instead of 4*(16*16)=1024

This comes with no significant encoder performance penalty based on tests
 against subset2 using intra predictors trained on subset1 + subset3.

Normal block switching:

            RATE     DSNR
    PSNR -0.020556 0.0014298
 PSNRHVS  0.38113 -0.026767
    SSIM  1.3508  -0.056021
FASTSSIM  0.63636 -0.019561

Only 4x4 blocks:

            RATE     DSNR
    PSNR  0.0      0.0
 PSNRHVS  0.0      0.0
    SSIM  0.0      0.0
FASTSSIM  0.0      0.0

Only 8x8 blocks:

            RATE     DSNR
    PSNR -0.26300  0.016693
 PSNRHVS  0.41150 -0.028184
    SSIM -0.27169  0.012379
FASTSSIM  0.53729 -0.011246

Only 16x16 blocks:

            RATE     DSNR
    PSNR -0.34254  0.018875
 PSNRHVS -0.19343  0.014677
    SSIM  1.1356  -0.045811
FASTSSIM -0.26803 -0.0011219