Elements not rendered properly when taking screenshot

[Alevsk]

Hi, this question is directly related to #138, the following snippet code works

func Screenshot(win *pixelgl.Window) {
	fmt.Println("taking screenshot...")

	f, err := os.Create("screenshot.png")
	if err != nil {
		panic(err)
	}
	defer f.Close()
	img := pixel.PictureDataFromPicture(win)
	png.Encode(f, img.Image())

	fmt.Println("done")
}

However some times when generating the screenshots some elements are not fully rendered in the image, ie:

In my specific scenario code looks almost the same with some minor differences:

func GenerateImage() {
	f, err := os.Create("screenshot.png")
	if err != nil {
		panic(err)
	}
	defer f.Close()
	img := pixel.PictureDataFromPicture(win)
	err = png.Encode(f, img.Image())
	if err != nil {
		panic(err)
	}
	fmt.Println("done")
}

win is a global variable var win *pixelgl.Window and I'm invoking the GenerateImage() from a different thread than the main thread, I'm wondering if that could be causing the problem and if there's a way to freeze or make win wait for all elements to be rendered before pixel.PictureDataFromPicture(win) is executed.