This fixes a bug where the image would not be loaded if the user setup a
keybinding which advances multiple slides at once. Then the preloading for the
previous/next slide in advance() would not catch that case and instead of an
image the text version (`@filename`) would be rendered on the slide.
if (slides[idx].img)
slides[idx].img->state &= ~(DRAWN | SCALED);
idx = new_idx;
+ ffload(&slides[idx]);
xdraw();
if (slidecount > idx + 1)
ffload(&slides[idx + 1]);