backends/screen-cast: Pass pixel format to DMA-BUF constructor

Following the previous commit, do the same function parameter
treatment to meta_screen_cast_create_dma_buf_handle().

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3175>

src/backends/meta-screen-cast-stream-src.c

@@ -1061,6 +1061,7 @@ on_stream_add_buffer (void             *data,
     {
       dmabuf_handle =
         meta_screen_cast_create_dma_buf_handle (screen_cast,
+                                                COGL_PIXEL_FORMAT_BGRX_8888,
                                                 priv->video_format.size.width,
                                                 priv->video_format.size.height);
     }