Add a speed feature for screen content type in rtc

This speed feature filters blocks so that less blocks will go
through the palette search path.

Disabling this feature leads to better coding gain but higher
key frame encoding time.

For speed 10 and 11, the feature is turned off for faster
key frame encoding.

For rtc_screen test set, if disabling the feature, the coding
performance is
              psnr        ssim        vmaf        encoding_time
Speed 10    -1.334%     -2.233%     -1.420%        -0.261%
Speed 11    -2.610%     -4.344%     -2.601%        0.207%

Change-Id: I0d2a729c806f7b2780c749fa6df169fbeb2758b8

av1/encoder/nonrd_pickmode.c

diff --git a/av1/encoder/nonrd_pickmode.c b/av1/encoder/nonrd_pickmode.c
index ed64056..d08e9c0 100644
--- a/av1/encoder/nonrd_pickmode.c
+++ b/av1/encoder/nonrd_pickmode.c
@@ -23,6 +23,7 @@
 #include "av1/encoder/model_rd.h"
 #include "av1/encoder/motion_search_facade.h"
 #include "av1/encoder/nonrd_opt.h"
+#include "av1/encoder/palette.h"
 #include "av1/encoder/reconinter_enc.h"
 #include "av1/encoder/var_based_part.h"
 
@@ -1655,11 +1656,15 @@
 
   // Try palette if it's enabled.
   bool try_palette =
-      (!args.prune_mode_based_on_sad || best_sad_norm > thresh_sad) &&
-      cpi->oxcf.tool_cfg.enable_palette && bsize <= BLOCK_16X16 &&
-      x->source_variance > 200 &&
+      cpi->oxcf.tool_cfg.enable_palette &&
       av1_allow_palette(cpi->common.features.allow_screen_content_tools,
                         mi->bsize);
+  if (cpi->sf.rt_sf.prune_screen_palette_search) {
+    bool prune =
+        (!args.prune_mode_based_on_sad || best_sad_norm > thresh_sad) &&
+        bsize <= BLOCK_16X16 && x->source_variance > 200;
+    try_palette &= prune;
+  }
   if (try_palette) {
     const TxfmSearchInfo *txfm_info = &x->txfm_search_info;
     const unsigned int intra_ref_frame_cost = 0;

av1/encoder/speed_features.c

diff --git a/av1/encoder/speed_features.c b/av1/encoder/speed_features.c
index 6719866..6fd01d8 100644
--- a/av1/encoder/speed_features.c
+++ b/av1/encoder/speed_features.c
@@ -1579,6 +1579,7 @@
       sf->rt_sf.nonrd_aggressive_skip = 1;
       sf->rt_sf.thresh_active_maps_skip_lf_cdef = 90;
       sf->rt_sf.hybrid_intra_pickmode = 0;
+      sf->rt_sf.prune_screen_palette_search = true;
       sf->rt_sf.prune_intra_mode_using_best_sad_so_far = true;
     }
     if (speed >= 11) {
@@ -2253,6 +2254,7 @@
   rt_sf->use_nonrd_filter_search = 0;
   rt_sf->use_simple_rd_model = 0;
   rt_sf->hybrid_intra_pickmode = 0;
+  rt_sf->prune_screen_palette_search = false;
   rt_sf->source_metrics_sb_nonrd = 0;
   rt_sf->overshoot_detection_cbr = NO_DETECTION;
   rt_sf->check_scene_detection = 0;

av1/encoder/speed_features.h

diff --git a/av1/encoder/speed_features.h b/av1/encoder/speed_features.h
index d6b2949..300d7e1 100644
--- a/av1/encoder/speed_features.h
+++ b/av1/encoder/speed_features.h
@@ -1629,6 +1629,12 @@
   // 2 : use rd for bsize < 16x16 and src var >= 101, nonrd otherwise
   int hybrid_intra_pickmode;
 
+  // Filter blocks by certain criteria such as SAD, source variance, such that
+  // fewer blocks will go through the palette search.
+  // For screen content types, enable this feature reduces key frame encoding
+  // time. Disabling it leads to better compression efficiency.
+  bool prune_screen_palette_search;
+
   // Compute variance/sse on source difference, prior to encoding superblock.
   int source_metrics_sb_nonrd;