func (w *Window) getCell(x, y int) *Cell {
	pos := x + (y * w.x)
	if pos < len(w.backingStore) {
		return &w.backingStore[pos]
	}
	return nil
}
func (w *Window) getCell(x, y int) *Cell {
	pos := x + (y * w.x)
	if pos < len(w.backingStore) {
		return &w.backingStore[pos]
	}
	return nil
}
// getCell returns the content of the window cell at the x and y coordinate.
// getCell shall be called from queue context.