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Modify non-rd intra mode checking

Speed 6 uses small tx size, namely 8x8. max_intra_bsize needs to
be modified accordingly to ensure valid intra mode checking.
Borg test on RTC set showed an overall PSNR gain of 0.335% in speed
-6.

This also changes speed -5 encoding by allowing DC_PRED checking
for block32x32. Borg test on RTC set showed a slight PSNR gain of
0.145%, and no noticeable speed change.

Change-Id: I1502978d8fbe265b3bb235db0f9c35ba0703cd45

--- a/vp9/encoder/vp9_pickmode.c

+++ b/vp9/encoder/vp9_pickmode.c

@@ -469,7 +469,7 @@

   // Perform intra prediction search, if the best SAD is above a certain

   // threshold.

   if (!x->skip && best_rd > inter_mode_thresh &&

-      bsize < cpi->sf.max_intra_bsize) {

+      bsize <= cpi->sf.max_intra_bsize) {

     for (this_mode = DC_PRED; this_mode <= DC_PRED; ++this_mode) {

       vp9_predict_intra_block(xd, 0, b_width_log2(bsize),

                               mbmi->tx_size, this_mode,

--- a/vp9/encoder/vp9_speed_features.c

+++ b/vp9/encoder/vp9_speed_features.c

@@ -270,6 +270,10 @@

     sf->source_var_thresh = 360;

     sf->tx_size_search_method = USE_TX_8X8;

+    // TODO(yunqingwang): max_intra_bsize is used to decide if DC_PRED mode

+    // is checked for a partition block. Later, we can try to allow large

+    // partitions to do intra mode checking.

+    sf->max_intra_bsize = BLOCK_8X8;

   }

   if (speed >= 7) {