Fix the broken directional intra prediction modes and re-enable them (#1063)

Fix the bug that PRED_45 works incorrectly with vertically elongated partition.

   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-0.5136 | -0.5447 | -0.4921 |  -0.5751 | -0.5807 | -0.5773 |    -0.4561