app,io/system: [API] add StageInactive when window is not in focus

Now, Gio will send one system.StageEvent with system.StageInactive when
the window is not active. It is implemented on macOS and Windows.

This change is not fully backward compatible, if your code compares
the Stage (`stage < system.StageRunning`), you need to consider
the new system.StageInactive.

Signed-off-by: inkeliz <inkeliz@inkeliz.com>

app/window.go

@@ -429,7 +429,7 @@ func (w *Window) driverDefer(f func(d driver)) {
 
 func (w *Window) updateAnimation(d driver) {
 	animate := false
-	if w.stage >= system.StageRunning && w.hasNextFrame {
+	if w.stage >= system.StageInactive && w.hasNextFrame {
 		if dt := time.Until(w.nextFrame); dt <= 0 {
 			animate = true
 		} else {
@@ -829,7 +829,7 @@ func (w *Window) processEvent(d driver, e event.Event) bool {
 	}
 	switch e2 := e.(type) {
 	case system.StageEvent:
-		if e2.Stage < system.StageRunning {
+		if e2.Stage < system.StageInactive {
 			if w.gpu != nil {
 				w.ctx.Lock()
 				w.gpu.Release()
@@ -845,7 +845,7 @@ func (w *Window) processEvent(d driver, e event.Event) bool {
 		if e2.Size == (image.Point{}) {
 			panic(errors.New("internal error: zero-sized Draw"))
 		}
-		if w.stage < system.StageRunning {
+		if w.stage < system.StageInactive {
 			// No drawing if not visible.
 			break
 		}