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/flex.go

@@ -173,7 +173,7 @@ func (f *Flex) Layout(gtx *Context, children ...FlexChild) {
 		var stack op.StackOp
 		stack.Push(gtx.Ops)
 		op.TransformOp{}.Offset(toPointF(axisPoint(f.Axis, mainSize, cross))).Add(gtx.Ops)
-		child.macro.Add(gtx.Ops)
+		child.macro.Add()
 		stack.Pop()
 		mainSize += axisMain(f.Axis, dims.Size)
 		if i < len(children)-1 {