Do sub-pixel motion search in up-sampled reference frames

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. This
largely improved the motion search precision, and thus, improved
the compression quality. There was no change in decoder side.

Borg test and speed test results:
1. On derflr set,
Overall PSNR gain: 1.306%, and SSIM gain: 1.512%.
Average speed loss on derf set was 6.0%.
2. On stdhd set,
Overall PSNR gain: 0.754%, and SSIM gain: 0.814%.
On hevchd set,
Overall PSNR gain: 0.465%, and SSIM gain: 0.527%.
Speed loss on HD clips was 3.5%.

Change-Id: I300ebaafff57e88914f3dedc8784cb21d316b04f

vp10/encoder/mcomp.h

@@ -116,7 +116,11 @@ typedef int (fractional_mv_step_fp) (
     int *mvjcost, int *mvcost[2],
     int *distortion, unsigned int *sse1,
     const uint8_t *second_pred,
+#if CONFIG_AFFINE_MOTION
+    int w, int h, int use_upsampled_ref);
+#else
     int w, int h);
+#endif
 
 extern fractional_mv_step_fp vp10_find_best_sub_pixel_tree;
 extern fractional_mv_step_fp vp10_find_best_sub_pixel_tree_pruned;