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/example_test.go

@@ -103,6 +103,30 @@ func ExampleStack() {
 	// Expand: {(50,50) (100,100)}
 }
 
+func ExampleBackground() {
+	gtx := layout.Context{
+		Ops: new(op.Ops),
+		Constraints: layout.Constraints{
+			Max: image.Point{X: 100, Y: 100},
+		},
+	}
+
+	layout.Background{}.Layout(gtx,
+		// Force widget to the same size as the second.
+		func(gtx layout.Context) layout.Dimensions {
+			fmt.Printf("Expand: %v\n", gtx.Constraints)
+			return layoutWidget(gtx, 10, 10)
+		},
+		// Rigid 50x50 widget.
+		func(gtx layout.Context) layout.Dimensions {
+			return layoutWidget(gtx, 50, 50)
+		},
+	)
+
+	// Output:
+	// Expand: {(50,50) (100,100)}
+}
+
 func ExampleList() {
 	gtx := layout.Context{
 		Ops: new(op.Ops),