mod/mpi: improve slideshow stopping
mod/mpi: improve slideshow stopping

The prior approach to stopping the slideshow was unreliable (e.g.
manually jumping to another file didn't always stop the slideshow.)

While this commit doesn't fix the problem completely, it significantly
reduces race chances.

Additionally, it removes the delay between slideshow start and moving to
the first next file.

rr-/dotfiles,rr-/dotfiles,rr-/dotfiles