layout: add Background

It's relatively common to create a widget and then add a background to
it. Using layout.Stack causes bunch of heap allocs, which we would like
to avoid whenever we can.

This adds layout.Background which is roughly the same as:

    layout.Stack{Alignment: layout.C}.Layout(gtx,
    	layout.Expanded(background),
    	layout.Stacked(widget)
    )

goos: windows
goarch: amd64
pkg: gioui.org/layout
cpu: AMD Ryzen Threadripper 2950X 16-Core Processor
     │    Stack     │             Background              │
     │    sec/op    │   sec/op     vs base                │
*-32   203.80n ± 1%   83.36n ± 3%  -59.09% (p=0.000 n=10)

     │   Stack    │             Background             │
     │    B/op    │   B/op     vs base                 │
*-32   48.00 ± 0%   0.00 ± 0%  -100.00% (p=0.000 n=10)

     │   Stack    │             Background              │
     │ allocs/op  │ allocs/op   vs base                 │
*-32   2.000 ± 0%   0.000 ± 0%  -100.00% (p=0.000 n=10)

Signed-off-by: Egon Elbre <egonelbre@gmail.com>

layout/stack.go

@@ -118,3 +118,36 @@ func (s Stack) Layout(gtx Context, children ...StackChild) Dimensions {
 		Baseline: baseline,
 	}
 }
+
+// Background lays out single child widget on top of a background,
+// centering, if necessary.
+type Background struct{}
+
+// Layout a widget and then add a background to it.
+func (Background) Layout(gtx Context, background, widget Widget) Dimensions {
+	macro := op.Record(gtx.Ops)
+	wdims := widget(gtx)
+	baseline := wdims.Baseline
+	call := macro.Stop()
+
+	cgtx := gtx
+	cgtx.Constraints.Min = gtx.Constraints.Constrain(wdims.Size)
+	bdims := background(cgtx)
+
+	if bdims.Size != wdims.Size {
+		p := image.Point{
+			X: (bdims.Size.X - wdims.Size.X) / 2,
+			Y: (bdims.Size.Y - wdims.Size.Y) / 2,
+		}
+		baseline += (bdims.Size.Y - wdims.Size.Y) / 2
+		trans := op.Offset(p).Push(gtx.Ops)
+		defer trans.Pop()
+	}
+
+	call.Add(gtx.Ops)
+
+	return Dimensions{
+		Size:     bdims.Size,
+		Baseline: baseline,
+	}
+}