Allow for 4x4 prediction blocks for key frame, speed 6.

For key frame under variance source partition: 4x4 prediction blocks
may be selected when variance of 8x8 block is very high (threshold is set fairly high for now).

Testing on some RTC clips shows this helps to reduce some ringing artifacts on key frame.
Encoded key frame size increases about ~10%. Key frame PSNR increases about ~0.1-0.2dB.

Change-Id: I56e203fac32ea6ef69897fb3ea269c59cb50d174

vp9/encoder/vp9_encodeframe.c

@@ -423,8 +423,9 @@ static int set_vt_partitioning(VP9_COMP *cpi,
 
   if (cm->frame_type == KEY_FRAME) {
     bsize_ref = BLOCK_8X8;
-    // Choose lower thresholds for key frame variance to favor split.
-    threshold_bsize_ref = threshold >> 1;
+    // Choose lower thresholds for key frame variance to favor split, but keep
+    // threshold for splitting to 4x4 block still fairly high for now.
+    threshold_bsize_ref = threshold << 2;
     threshold_low = threshold >> 2;
   }
 
@@ -646,8 +647,6 @@ static void choose_partitioning(VP9_COMP *cpi,
             for (k = 0; k < 4; ++k) {
               const int x8_idx = (k & 1);
               const int y8_idx = (k >> 1);
-              // TODO(marpan): Allow for setting 4x4 partition on key frame.
-              /*
               if (cm->frame_type == KEY_FRAME) {
                 if (!set_vt_partitioning(cpi, xd,
                                          &vt.split[i].split[j].split[k],
@@ -660,12 +659,11 @@ static void choose_partitioning(VP9_COMP *cpi,
                                    BLOCK_4X4);
                 }
               } else {
-              */
                 set_block_size(cpi, xd,
                                (mi_row + y32_idx + y16_idx + y8_idx),
                                (mi_col + x32_idx + x16_idx + x8_idx),
                                BLOCK_8X8);
-              // }
+               }
             }
           }
         }