ui: move macro recording from Ops to MacroOp

Move the Record and Stop methods from Ops to MacroOp itself.

Before this change, Ops.Stop stopped the recording of the most
recent macro, which could be a different macro than intended.
After this change, there is no such confusion.

As a bonus, the Ops API becomes less cluttered.

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

ui/layout/stack.go

@@ -11,6 +11,7 @@ import (
 type Stack struct {
 	Alignment Direction
 
+	macro       ui.MacroOp
 	ops         *ui.Ops
 	constrained bool
 	cs          Constraints
@@ -54,7 +55,7 @@ func (s *Stack) begin() {
 		panic("must End before adding a child")
 	}
 	s.begun = true
-	s.ops.Record()
+	s.macro.Record(s.ops)
 }
 
 func (s *Stack) Rigid() Constraints {
@@ -71,7 +72,7 @@ func (s *Stack) Expand() Constraints {
 }
 
 func (s *Stack) End(dims Dimens) StackChild {
-	b := s.ops.Stop()
+	s.macro.Stop()
 	s.begun = false
 	if w := dims.Size.X; w > s.maxSZ.X {
 		s.maxSZ.X = w
@@ -84,7 +85,7 @@ func (s *Stack) End(dims Dimens) StackChild {
 			s.baseline = b
 		}
 	}
-	return StackChild{b, dims}
+	return StackChild{s.macro, dims}
 }
 
 func (s *Stack) Layout(children ...StackChild) Dimens {