op: remove operation list argument from MacroOp.Add

The ability to invoke other operation lists belongs in the new CallOp.

While we're here, make MacroOp.Add use a pointer receiver to match the
other methods.

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

layout/layout.go

@@ -173,7 +173,7 @@ func (a Align) Layout(gtx *Context, w Widget) {
 	var stack op.StackOp
 	stack.Push(gtx.Ops)
 	op.TransformOp{}.Offset(toPointF(p)).Add(gtx.Ops)
-	macro.Add(gtx.Ops)
+	macro.Add()
 	stack.Pop()
 	gtx.Dimensions = Dimensions{
 		Size:     sz,