app: [API] remove StageEvent and Stage

StageEvent served only redundant purposes:

- To detect whether the window has focus. That is covered by
  key.FocusEvent.
- To detect whether the window is currently visible. That is covered by
  the absence or presence of FrameEvents.
- To detect when the window native handle is valid. That is
  covered by ViewEvent.

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

app/os_wayland.go

@@ -199,7 +199,7 @@ type window struct {
 		dir            f32.Point
 	}
 
-	stage             Stage
+	configured        bool
 	lastFrameCallback *C.struct_wl_callback
 
 	animating bool
@@ -549,7 +549,7 @@ func gio_onXdgSurfaceConfigure(data unsafe.Pointer, wmSurf *C.struct_xdg_surface
 	w := callbackLoad(data).(*window)
 	w.serial = serial
 	C.xdg_surface_ack_configure(wmSurf, serial)
-	w.setStage(StageRunning)
+	w.configured = true
 	w.draw(true)
 }
 
@@ -1738,10 +1738,7 @@ func (w *window) updateOutputs() {
 		C.wl_surface_set_buffer_scale(w.surf, C.int32_t(w.scale))
 		w.draw(true)
 	}
-	if !found {
-		w.setStage(StagePaused)
-	} else {
-		w.setStage(StageRunning)
+	if found {
 		w.draw(true)
 	}
 }
@@ -1755,6 +1752,9 @@ func (w *window) getConfig() (image.Point, unit.Metric) {
 }
 
 func (w *window) draw(sync bool) {
+	if !w.configured {
+		return
+	}
 	w.flushScroll()
 	size, cfg := w.getConfig()
 	if cfg == (unit.Metric{}) {
@@ -1785,14 +1785,6 @@ func (w *window) draw(sync bool) {
 	})
 }
 
-func (w *window) setStage(s Stage) {
-	if s == w.stage {
-		return
-	}
-	w.stage = s
-	w.ProcessEvent(StageEvent{Stage: s})
-}
-
 func (w *window) display() *C.struct_wl_display {
 	return w.disp.disp
 }