app/headless: handle error from Framebuffer.ReadPixels

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

app/headless/headless.go

@@ -116,12 +116,15 @@ func (w *Window) Frame(frame *op.Ops) {
 // Screenshot returns an image with the content of the window.
 func (w *Window) Screenshot() (*image.RGBA, error) {
 	img := image.NewRGBA(image.Rectangle{Max: w.size})
-	contextDo(w.ctx, func() error {
+	err := contextDo(w.ctx, func() error {
 		return w.fbo.ReadPixels(
 			image.Rectangle{
 				Max: image.Point{X: w.size.X, Y: w.size.Y},
 			}, img.Pix)
 	})
+	if err != nil {
+		return nil, err
+	}
 	// Flip image in y-direction. OpenGL's origin is in the lower
 	// left corner.
 	row := make([]uint8, img.Stride)