Back off overlay frame rate when all timing tests have fired (saves CPU)

Closes #225 github issue (Jay Freeman).

src/frontend/terminaloverlay.cc

@@ -285,8 +285,10 @@ int OverlayManager::wait_time( void )
     next_expiry = 1000;
   }
 
-  if ( predictions.active() && ( next_expiry > 20 ) ) {
-    next_expiry = 20;
+  if ( predictions.timing_tests_necessary()
+       && predictions.active()
+       && ( next_expiry > 50 ) ) {
+    next_expiry = 50;
   }
 
   return next_expiry;

src/frontend/terminaloverlay.h

@@ -214,6 +214,11 @@ namespace Overlay {
 
     bool active( void ) const;
 
+    bool timing_tests_necessary( void ) const {
+      /* Are there any timing-based triggers that haven't fired yet? */
+      return !( glitch_trigger && flagging );
+    }
+
     void set_local_frame_sent( uint64_t x ) { local_frame_sent = x; }
     void set_local_frame_acked( uint64_t x ) { local_frame_acked = x; }
     void set_local_frame_late_acked( uint64_t x ) { local_frame_late_acked = x; }