Let Movie only assemble animation (or make a dry run).

When I want to test how **gmt movie** is using `ffmpeg` to assemble all the frames for a movie, I need to run gmt movie from the scratch. This means that all the frames are created again and this could take a lot of time.

So, it would good to have a option to just ask **gmt movie** to assemble the images directly. There is now an option to only create the frames (-Fpng).

I think that an alternative would be to let **movie** to make a dry run so I could see the `ffpmeg` command [in the terminal](https://docs.generic-mapping-tools.org/dev/movie.html#remaking-movie-with-existing-png-frames).