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_test.go

@@ -0,0 +1,66 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package layout
+
+import (
+	"image"
+	"testing"
+
+	"gioui.org/op"
+)
+
+func BenchmarkStack(b *testing.B) {
+	gtx := Context{
+		Ops: new(op.Ops),
+		Constraints: Constraints{
+			Max: image.Point{X: 100, Y: 100},
+		},
+	}
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		gtx.Ops.Reset()
+
+		Stack{}.Layout(gtx,
+			Expanded(emptyWidget{
+				Size: image.Point{X: 60, Y: 60},
+			}.Layout),
+			Stacked(emptyWidget{
+				Size: image.Point{X: 30, Y: 30},
+			}.Layout),
+		)
+	}
+}
+
+func BenchmarkBackground(b *testing.B) {
+	gtx := Context{
+		Ops: new(op.Ops),
+		Constraints: Constraints{
+			Max: image.Point{X: 100, Y: 100},
+		},
+	}
+	b.ReportAllocs()
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		gtx.Ops.Reset()
+
+		Background{}.Layout(gtx,
+			emptyWidget{
+				Size: image.Point{X: 60, Y: 60},
+			}.Layout,
+			emptyWidget{
+				Size: image.Point{X: 30, Y: 30},
+			}.Layout,
+		)
+	}
+}
+
+type emptyWidget struct {
+	Size image.Point
+}
+
+func (w emptyWidget) Layout(gtx Context) Dimensions {
+	return Dimensions{Size: w.Size}
+}