font/opentype,text,widget: use clip.Op for text shapes, not a macro

This change avoids a macro wrapping every text shape, and prepares text
shaping for scoped clip operations.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

widget/editor.go

@@ -180,7 +180,7 @@ type SelectEvent struct{}
 
 type line struct {
 	offset         image.Point
-	clip           op.CallOp
+	clip           clip.Op
 	selected       bool
 	selectionYOffs int
 	selectionSize  image.Point