CanvasCloser is now actually a valid type

canvas/canvas.go

@@ -2,6 +2,7 @@
 // primitives.
 package canvas
 
+import "io"
 import "image"
 import "image/draw"
 import "image/color"
@@ -80,7 +81,7 @@ type Canvas interface {
 // CanvasCloser is a canvas that can be closed. Anything that receives a
 // CanvasCloser must close it after use.
 type CanvasCloser interface {
-	CanvasCloser
+	Canvas
 	io.Closer
 }