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/stack.go

@@ -97,7 +97,7 @@ func (s *Stack) Layout(gtx *Context, children ...StackChild) {
 		var stack op.StackOp
 		stack.Push(gtx.Ops)
 		op.TransformOp{}.Offset(toPointF(p)).Add(gtx.Ops)
-		ch.macro.Add(gtx.Ops)
+		ch.macro.Add()
 		stack.Pop()
 		if baseline == 0 {
 			if b := ch.dims.Baseline; b != 0 {