Added a rectangle method to Canvas

canvas/canvas.go

@@ -32,8 +32,11 @@ type StrokeAlign int; const (
 // have multiple pens associated with it, each maintaining their own drawing
 // context.
 type Pen interface {
-	// Draw draws a path
-	Draw (points ...image.Point)
+	// Rectangle draws a rectangle
+	Rectangle (image.Rectangle)
+
+	// Path draws a path
+	Path (points ...image.Point)
 
 	Closed       (bool)        // if the path is closed
 	Cap          (Cap)         // line cap stype