window.go
func (w *Window) setCell(x, y int, c Cell) {
	c.dirty = true
	pos := x + (y * w.x)
	if pos < len(w.backingStore) {
		w.backingStore[pos] = c
	}
}
func (w *Window) setCell(x, y int, c Cell) {
	c.dirty = true
	pos := x + (y * w.x)
	if pos < len(w.backingStore) {
		w.backingStore[pos] = c
	}
}
// setCell sets the content of the window cell at the x and y coordinate.
// setCell shall be called from queue context.