Assert screen closed on destroy

The destruction order is important, but tricky, because the screen is
open/close by the decoder, but destroyed by scrcpy.c on the main thread.

Add assertions to guarantee that the screen is not destroyed before
being closed.

app/src/screen.h

@@ -15,6 +15,10 @@
 struct screen {
     struct sc_frame_sink frame_sink; // frame sink trait
 
+#ifndef NDEBUG
+    bool open; // track the open/close state to assert correct behavior
+#endif
+
     struct video_buffer vb;
     struct fps_counter *fps_counter;