add clipping

pitfall/pitfall/vector.rkt

@@ -29,6 +29,7 @@
      fill
      stroke
      fillAndStroke
+     clip
      transform
      translate
      scale)))
@@ -210,6 +211,11 @@
         (+ (* n12 mdx) (* n22 mdy) ndy)))
 
 
+(define/contract (clip this [rule #f])
+  (() ((or/c string? #f)) . ->*m . object?)
+  (send this addContent (string-append "W" (_windingRule rule) " n")))
+
+
 (define/contract (transform this m11 m12 m21 m22 mdx mdy)
   (number? number? number? number? number? number? . ->m . object?)
   (define new-ctm (list m11 m12 m21 m22 mdx mdy))