Displays images in a looping slideshow.

See also https://github.com/scy/card10slide.

Images need to be in PPM "raw" or "binary" or "P6" (not "ASCII") format, 8×8×8 bit RGB color, 160×80 pixels.
You can use for example GIMP or ImageMagick to create these images.
Place them in a directory named `slideshow` in your card10 main folder and make sure they end in `.ppm` (case sensitive!).

Error handling could use some improvement, but I'm tired.