Allow mode search breakout at very low prediction errors

In model_rd_for_sb function, the spatial domain SSE and variance
are checked to see if transform coefficients are quantized to 0.
Besides that, this patch adds another set of thresholds that are
much more strict. These thresholds are used to conduct a partition
block level check to measure if all its TX blocks are skippable
for YUV planes. If it is true, x->skip is set for this partition
block, and thus its mode search is terminated.

This speeds up the encoding at very low prediction error case,
such as screen sharing application. This patch covers what
rd_encode_breakout_test() does, so that function is removed.

Borg test at speed 3 shows:
For stdhd set, psnr: +0.008%, ssim: +0.014%;
For derf set, psnr: +0.018%, ssim: +0.025%.
No noticeable speed change.

Change-Id: I4e5f15cf10016a282a68e35175ff854b28195944

vp9/encoder/vp9_firstpass.h

@@ -109,8 +109,6 @@ typedef struct {
 
   int kf_zeromotion_pct;
   int last_kfgroup_zeromotion_pct;
-  int gf_zeromotion_pct;
-
   int active_worst_quality;
 
   GF_GROUP gf_group;