Refactor variance aq.

Explicitly signal when the segment map is being refreshed when
using VARIANE_AQ. This simplifies decisions about when the segment id
needs to be set from the previous segment map vs based on the current
variance.

Change-Id: Ieb12c950e9cfbc3f53f4d184880071dea805563c

vp10/encoder/encoder.c

@@ -296,6 +296,8 @@ static void setup_frame(VP10_COMP *cpi) {
     vp10_zero(cpi->interp_filter_selected[0]);
   }
 
+  cpi->vaq_refresh = 0;
+
   set_sb_size(cm, select_sb_size(cpi));
 }