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Improve skip_txfm thresholds in the non-rd mode selection

Modified the thresholds of deciding whether or not to skip
the transforms in model_rd_for_sb_y(). Used zbin[] instead
of dequant[] to be more precise. Also, modified the checking
coditions.

Rtc set borg test results (at speed 6) showed:
average PSNR gain: 0.138%, overall PSNR gain: 0.158%,
and SSIM gain: 0.177%.

The data rate test was modified slightly as suggested by
Marco.

Change-Id: Ieaf633ab77f4838cb3c45cf69065b29d55f8ae6c

--- a/test/datarate_test.cc

+++ b/test/datarate_test.cc

@@ -540,7 +540,7 @@

         << " The first dropped frame for drop_thresh " << i

         << " > first dropped frame for drop_thresh "

         << i - kDropFrameThreshTestStep;

-    ASSERT_GE(num_drops_, last_num_drops)

+    ASSERT_GE(num_drops_, last_num_drops * 0.90)

         << " The number of dropped frames for drop_thresh " << i

         << " < number of dropped frames for drop_thresh "

         << i - kDropFrameThreshTestStep;

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

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

@@ -216,6 +216,8 @@

   int64_t dist;

   struct macroblock_plane *const p = &x->plane[0];

   struct macroblockd_plane *const pd = &xd->plane[0];

+  const int64_t dc_thr = p->quant_thred[0] >> 6;

+  const int64_t ac_thr = p->quant_thred[1] >> 6;

   const uint32_t dc_quant = pd->dequant[0];

   const uint32_t ac_quant = pd->dequant[1];

   unsigned int var = cpi->fn_ptr[bsize].vf(p->src.buf, p->src.stride,

@@ -223,12 +225,14 @@

   *var_y = var;

   *sse_y = sse;

-  if (sse < dc_quant * dc_quant >> 6)

-    x->skip_txfm[0] = 1;

-  else if (var < ac_quant * ac_quant >> 6)

+  x->skip_txfm[0] = 0;

+  // Check if all ac coefficients can be quantized to zero.

+  if (var < ac_thr || var == 0) {

     x->skip_txfm[0] = 2;

-  else

-    x->skip_txfm[0] = 0;

+    // Check if dc coefficient can be quantized to zero.

+    if (sse - var < dc_thr || sse == var)

+      x->skip_txfm[0] = 1;

+  }

   if (cpi->common.tx_mode == TX_MODE_SELECT) {

     if (sse > (var << 2))