Fix preview not working after switching windows

src/io_pipeline.c

@@ -375,6 +375,7 @@ on_frame(MPBuffer buffer, void *_data)
         // an image, in which case the flash window may be active instead of this
         // window
         if (!check_window_active() && state_io.captures_remaining == 0) {
+                mp_io_pipeline_release_buffer(buffer.index);
                 return;
         }