Add RDO decision for partition splitting (#207)

* Add RDO decision for partitioning

master (-s=2) -> current (-s=2)

   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-7.4095 |     N/A |     N/A |  -6.8990 | -9.0320 | -8.7738 |   -17.4584

master (-s=3) -> current (-s=2)

   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-5.1051 |  9.5605 |     N/A |  -5.1176 | -5.3494 | -5.1293 |     1.4963

master (-s=3) -> current (-s=0)

   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-7.7416 |     N/A |     N/A |  -7.1631 | -7.8223 | -7.2918 |     3.6073

* Increase RD cost comparison precision

master (-s=3) -> current (-s=0)

   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-7.5800 |     N/A |     N/A |  -7.0594 | -7.8593 | -7.3970 |     4.0382

* Avoid creating a new struct for child modes in partition decisions

* Add comments in encode_partition()