app/internal/window: block Main until all windows are closed

With this change, the Wayland backend now supports multiple windows.

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

app/internal/window/os_x11.go

@@ -422,12 +422,16 @@ func newX11Window(gioWin Callbacks, opts *Options) error {
 	// make the window visible on the screen
 	C.XMapWindow(dpy, win)
 
+	// Increment window counter.
+	windowCounter <- +1
 	go func() {
+		defer func() {
+			windowCounter <- -1
+		}()
 		w.w.SetDriver(w)
 		w.setStage(system.StageRunning)
 		w.loop()
 		w.destroy()
-		close(mainDone)
 	}()
 	return nil
 }