app: [Windows] avoid deadlock when changing window configuration

Some window configurations lead to WM_SIZE messages when changed, which
leads to deadlock because our window proc is not re-entrant. Avoid the
issues by ignoring redundant messages.

Fixes gio#262

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

app/os_windows.go

@@ -444,8 +444,10 @@ func (w *window) Wakeup() {
 }
 
 func (w *window) setStage(s system.Stage) {
-	w.stage = s
-	w.w.Event(system.StageEvent{Stage: s})
+	if s != w.stage {
+		w.stage = s
+		w.w.Event(system.StageEvent{Stage: s})
+	}
 }
 
 func (w *window) draw(sync bool) {