rtc: Reduce thresholds for palette mode in nonrd

For screen content in nonrd_pickmode:
reduce the sse and source_variance thresholds to force
more palette mode testing.

This had good bdrate gains with small increase in
encode time. For speed 10 rtc_screen:
2% bdrate gain in ovr-psnr, ~0.7% IC avg. increase.
Several clips (buganizer, web_browsing) have ~4% bdrate gains.

Also observed reduction in some visual artifacts in
buganizer and web_browsing clips.

Change-Id: I183a5dae1f91c1229ae9217d24d0387c0a71157f

av1/encoder/nonrd_pickmode.c

diff --git a/av1/encoder/nonrd_pickmode.c b/av1/encoder/nonrd_pickmode.c
index 155c466..7754cd6 100644
--- a/av1/encoder/nonrd_pickmode.c
+++ b/av1/encoder/nonrd_pickmode.c
@@ -3266,8 +3266,8 @@
   if (cpi->oxcf.tune_cfg.content == AOM_CONTENT_SCREEN &&
       x->content_state_sb.source_sad_nonrd != kZeroSad &&
       bsize <= BLOCK_16X16) {
-    unsigned int thresh_sse = cpi->rc.high_source_sad ? 50000 : 500000;
-    unsigned int thresh_source_var = cpi->rc.high_source_sad ? 200 : 2000;
+    unsigned int thresh_sse = cpi->rc.high_source_sad ? 15000 : 250000;
+    unsigned int thresh_source_var = cpi->rc.high_source_sad ? 50 : 1000;
     unsigned int best_sse_inter_motion =
         (unsigned int)(search_state.best_rdc.sse >>
                        (b_width_log2_lookup[bsize] +