screenshot-layer: Fix rename() to final png before the file is flushed.

If the app exited before the thread was finished, then we might get an
incomplete png at the final filename.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/41299>

src/vulkan/screenshot-layer/screenshot.cpp

@@ -859,6 +859,11 @@ void *writePNG(void *data) {
    png_write_end(png, NULL);          // End image writing
    free(row_pointer);
 
+   // Close before renaming, to ensure the writes are flushed (in case of app
+   // exit without vkDestroyDevice() to join our thread).
+   fclose(file);
+   file = NULL;
+
    // Rename file, indicating completion, client should be
    // checking for the final file exists.
    if (rename(tmpFilename, filename) != 0 )