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/flex.go

@@ -14,6 +14,7 @@ type Flex struct {
 	MainAxisAlignment  MainAxisAlignment
 	CrossAxisAlignment CrossAxisAlignment
 
+	macro       ui.MacroOp
 	ops         *ui.Ops
 	cs          Constraints
 	mode        flexMode
@@ -74,7 +75,7 @@ func (f *Flex) begin(mode flexMode) {
 		panic("must End before adding a child")
 	}
 	f.mode = mode
-	f.ops.Record()
+	f.macro.Record(f.ops)
 }
 
 func (f *Flex) Rigid() Constraints {
@@ -107,7 +108,7 @@ func (f *Flex) End(dims Dimens) FlexChild {
 	if f.mode <= modeBegun {
 		panic("End called without an active child")
 	}
-	macro := f.ops.Stop()
+	f.macro.Stop()
 	sz := axisMain(f.Axis, dims.Size)
 	f.size += sz
 	if f.mode == modeRigid {
@@ -120,7 +121,7 @@ func (f *Flex) End(dims Dimens) FlexChild {
 	if b := dims.Baseline; b > f.maxBaseline {
 		f.maxBaseline = b
 	}
-	return FlexChild{macro, dims}
+	return FlexChild{f.macro, dims}
 }
 
 func (f *Flex) Layout(children ...FlexChild) Dimens {