The "render_stats" handler now includes the message line that was
written to stdout as the first arg (using `BKE_callback_exec_string`
instead of just `BKE_callback_exec_null`).
This can be useful to track render progress (and people were relying on
parsing stdout in the past).
Was a request in chat.
This also corrects possible output from the callback being written in
between message parts in `do_write_image_or_movie` (which was probably a
mistake in 93d5e106aa).
Philipp Oeser