Use small transform size in non-rd real-time mode

In non-rd real-time mode, choosing smaller transform size in
encoding gives better video quality and good speed gain than
choosing larger transform size. This patch set tx size search
method to ALLOW_8X8, which is better than using 4x4 or other
larger sizes.

Borg tests on rtc set at speed 6 showed significant gain on quality.
PSNR gain: 11.034% and SSIM gain: 15.466%.

The speed gain is 5% - 12% for <720p clips, and 2% - 7% for
720p clips.

Change-Id: If4dc74ed2df359346b059f47fb73b4a0193ec548

vp9/encoder/vp9_encodeframe.c

@@ -2418,6 +2418,8 @@ static TX_MODE select_tx_mode(const VP9_COMP *cpi) {
       return rd_opt->tx_select_threshes[frame_type][ALLOW_32X32] >
                  rd_opt->tx_select_threshes[frame_type][TX_MODE_SELECT] ?
                      ALLOW_32X32 : TX_MODE_SELECT;
+    } else if (cpi->sf.tx_size_search_method == USE_TX_8X8) {
+      return ALLOW_8X8;
     } else {
       unsigned int total = 0;
       int i;

vp9/encoder/vp9_speed_features.c

@@ -269,6 +269,8 @@ static void set_rt_speed_feature(VP9_COMP *cpi, SPEED_FEATURES *sf,
     sf->partition_search_type = SOURCE_VAR_BASED_PARTITION;
     sf->search_type_check_frequency = 50;
     sf->source_var_thresh = 360;
+
+    sf->tx_size_search_method = USE_TX_8X8;
   }
 
   if (speed >= 7) {

vp9/encoder/vp9_speed_features.h

@@ -58,7 +58,8 @@ typedef enum {
   USE_FULL_RD = 0,
   USE_LARGESTINTRA,
   USE_LARGESTINTRA_MODELINTER,
-  USE_LARGESTALL
+  USE_LARGESTALL,
+  USE_TX_8X8
 } TX_SIZE_SEARCH_METHOD;
 
 typedef enum {