Patch from Jon Wilson: added a home function

svn: r14698

collects/graphics/turtle-sig.ss

@@ -1,7 +1,7 @@
 #lang scheme/signature
 
 turtles
-clear
+clear home
 turn turn/radians
 move move-offset
 draw draw-offset

collects/graphics/turtle-unit.ss

@@ -227,6 +227,13 @@
     (set! lines-in-drawing null)
     (clear-window)))
 
+(define home
+  (lambda ()
+    (flip-icons)
+    (set! turtles-cache empty-cache)
+    (set! turtles-state (list clear-turtle))
+    (flip-icons)))
+
 ;; cache elements:
 (define-struct c-forward (distance))
 (define-struct c-turn (angle))