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.c

@@ -247,6 +247,9 @@ static bool
 screen_frame_sink_open(struct sc_frame_sink *sink) {
     struct screen *screen = DOWNCAST(sink);
     (void) screen;
+#ifndef NDEBUG
+    screen->open = true;
+#endif
 
     // nothing to do, the screen is already open on the main thread
     return true;
@@ -256,6 +259,9 @@ static void
 screen_frame_sink_close(struct sc_frame_sink *sink) {
     struct screen *screen = DOWNCAST(sink);
     (void) screen;
+#ifndef NDEBUG
+    screen->open = false;
+#endif
 
     // nothing to do, the screen lifecycle is not managed by the frame producer
 }
@@ -435,6 +441,10 @@ screen_init(struct screen *screen, struct fps_counter *fps_counter,
 
     screen->frame_sink.ops = &ops;
 
+#ifndef NDEBUG
+    screen->open = false;
+#endif
+
     return true;
 }
 
@@ -445,6 +455,9 @@ screen_show_window(struct screen *screen) {
 
 void
 screen_destroy(struct screen *screen) {
+#ifndef NDEBUG
+    assert(!screen->open);
+#endif
     av_frame_free(&screen->frame);
     SDL_DestroyTexture(screen->texture);
     SDL_DestroyRenderer(screen->renderer);

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;