Up-sampled the reference frames to 8 times in each dimension using the
8-tap interpolation filter. In sub-pixel motion search, use the up-sampled
reference frames to find the best matching blocks to increase the motion
search precision. This is enabled as a speed feature for speed 0 and
speed 1, and this is encoder-only improvement.
Overall PSNR: -1.456%(lowres); -0.430(hdres)
SSIM: -1.687(lowres); -0.551(hdres)