app/headless: implement headless windows

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

app/headless/headless_test.go

@@ -0,0 +1,41 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+package headless
+
+import (
+	"image"
+	"image/color"
+	"testing"
+
+	"gioui.org/f32"
+	"gioui.org/op"
+	"gioui.org/op/paint"
+)
+
+func TestHeadless(t *testing.T) {
+	sz := image.Point{X: 800, Y: 600}
+	w, err := NewWindow(sz.X, sz.Y)
+	if err != nil {
+		t.Skipf("headless windows not supported: %v", err)
+	}
+
+	col := color.RGBA{A: 0xff, R: 0xcc, G: 0xcc}
+	var ops op.Ops
+	paint.ColorOp{Color: col}.Add(&ops)
+	paint.PaintOp{Rect: f32.Rectangle{Max: f32.Point{
+		X: float32(sz.X),
+		Y: float32(sz.Y),
+	}}}.Add(&ops)
+	w.Frame(&ops)
+
+	img, err := w.Screenshot()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if isz := img.Bounds().Size(); isz != sz {
+		t.Errorf("got %v screenshot, expected %v", isz, sz)
+	}
+	if got := img.RGBAAt(0, 0); got != col {
+		t.Errorf("got color %v, expected %v", got, col)
+	}
+}