ui: make OpPush and OpPop explicit

We're about to allow OpBlock for invoking ops from multiple (cached)
Ops containers. To allow for drawing state changes to stick after
invoking such a cached block, we can't let OpBlock perform an implicit
save and restore of drawing state.

Instead, introduce OpPush and OpPop for explicit drawing state stack
management.

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

ui/layout/layout.go

@@ -97,13 +97,13 @@ func (in *Insets) Begin(ops *ui.Ops, cs Constraints) Constraints {
 			mcs.Height.Max = mcs.Height.Min
 		}
 	}
-	ops.Begin()
+	ui.OpPush{}.Add(ops)
 	ui.OpTransform{Transform: ui.Offset(toPointF(image.Point{X: l, Y: t}))}.Add(ops)
 	return mcs
 }
 
 func (in *Insets) End(ops *ui.Ops, dims Dimens) Dimens {
-	ops.End().Add(ops)
+	ui.OpPop{}.Add(ops)
 	t, r, b, l := int(math.Round(float64(in.Top))), int(math.Round(float64(in.Right))), int(math.Round(float64(in.Bottom))), int(math.Round(float64(in.Left)))
 	return Dimens{
 		Size:     in.cs.Constrain(dims.Size.Add(image.Point{X: r + l, Y: t + b})),
@@ -176,10 +176,10 @@ func (a *Align) End(ops *ui.Ops, dims Dimens) Dimens {
 	case SW, S, SE:
 		p.Y = sz.Y - dims.Size.Y
 	}
-	ops.Begin()
+	ui.OpPush{}.Add(ops)
 	ui.OpTransform{Transform: ui.Offset(toPointF(p))}.Add(ops)
 	block.Add(ops)
-	ops.End().Add(ops)
+	ui.OpPop{}.Add(ops)
 	return Dimens{
 		Size:     sz,
 		Baseline: dims.Baseline,