display: Add function to determine if display is blank.

display/display.c

@@ -439,6 +439,15 @@ queue_point(struct point *p)
     p->delay = d;
 }
 
+/*
+ * Return true if the display is blank, i.e. no active points in list.
+ */
+int
+display_is_blank(void)
+{
+    return head->next == head;
+}
+
 /*
  * here to to dynamically adjust interval for examination
  * of elapsed vs. simulated time, and fritter away

display/display.h

@@ -83,6 +83,11 @@ extern int display_scale(void);
  */
 extern int display_age(int,int);
 
+/*
+ * Return true if the display is blank.
+ */
+extern int display_is_blank(void);
+
 /*
  * display intensity levels.
  * always at least 8 (for VT11/VS60) -- may be mapped internally