rtc: Enable RTC reference strucutre for speed >= 6

Enable set_reference_structure for 1 pass RTC, for
speed >= 6. This bypasses the internal structure,
but the behavior explicitly set by the flag
set_reference_structure is very similar (mainly
fixes the golden update to be what was intended for
RTC, and provides flexibility for future adjustments).

bdrate gains on all clips for rtc_derf set:
speed 7: avg. ~1.1%, max ~4.8, a few clips over 3%.
speed 6: avg. ~0.35, max ~1.45.

No speed/fps change observed.

Change-Id: I25bdd6fa5282032ef40697f4226e310a0bdaeb1b

av1/encoder/ratectrl.c

diff --git a/av1/encoder/ratectrl.c b/av1/encoder/ratectrl.c
index 6b93e10..40cc509 100644
--- a/av1/encoder/ratectrl.c
+++ b/av1/encoder/ratectrl.c
@@ -1921,7 +1921,7 @@
   int target;
   // Turn this on to explicitly set the reference structure rather than
   // relying on internal/default structure.
-  const int set_reference_structure = 0;
+  const int set_reference_structure = 1;
   if (cpi->use_svc) {
     av1_update_temporal_layer_framerate(cpi);
     av1_restore_layer_context(cpi);
@@ -1996,8 +1996,7 @@
   }
   av1_rc_set_frame_target(cpi, target, cm->width, cm->height);
   rc->base_frame_target = target;
-  // For 1 layer nonrd allow option to set reference structure.
-  if (set_reference_structure && cpi->sf.use_nonrd_pick_mode &&
+  if (set_reference_structure && cpi->oxcf.speed >= 6 &&
       cm->number_spatial_layers == 1 && cm->number_temporal_layers == 1)
     set_reference_structure_one_pass_rt(cpi);
 }

test/rt_end_to_end_test.cc

diff --git a/test/rt_end_to_end_test.cc b/test/rt_end_to_end_test.cc
index 8edaf18..18a21aa 100644
--- a/test/rt_end_to_end_test.cc
+++ b/test/rt_end_to_end_test.cc
@@ -31,11 +31,11 @@
 std::unordered_map<std::string,
                    std::unordered_map<int, std::unordered_map<int, double>>>
     kPsnrThreshold = { { "park_joy_90p_8_420.y4m",
-                         { { 6, { { 0, 35.4 }, { 3, 36.3 } } },
+                         { { 6, { { 0, 35.4 }, { 3, 36.2 } } },
                            { 7, { { 0, 34.9 }, { 3, 35.8 } } },
                            { 8, { { 0, 35.0 }, { 3, 35.8 } } } } },
                        { "paris_352_288_30.y4m",
-                         { { 6, { { 0, 36.3 }, { 3, 36.7 } } },
+                         { { 6, { { 0, 36.2 }, { 3, 36.7 } } },
                            { 7, { { 0, 35.5 }, { 3, 36.0 } } },
                            { 8, { { 0, 36.0 }, { 3, 36.5 } } } } },
                        { "niklas_1280_720_30.y4m",