AV1 RT: adjust speed features for midres

With using altref and avoiding temporal variance shortcut on speed 8 on
midres the BDRate improvement is 3.7% overall on RTC with ~2=3% slowdown

Change-Id: I0e197a45ed3e2beaa6f78a05a5b321b99bba839b

av1/encoder/speed_features.c

diff --git a/av1/encoder/speed_features.c b/av1/encoder/speed_features.c
index 3ad2655..fb84022 100644
--- a/av1/encoder/speed_features.c
+++ b/av1/encoder/speed_features.c
@@ -314,6 +314,11 @@
     if (speed >= 9) {
       sf->rt_sf.use_modeled_non_rd_cost = 1;
     }
+  } else {
+    if (speed == 8 && !cpi->use_svc) {
+      sf->rt_sf.short_circuit_low_temp_var = 0;
+      sf->rt_sf.use_nonrd_altref_frame = 1;
+    }
   }
   if (!is_480p_or_larger) {
     if (speed == 7) {