Also call ffload on the current slide in advance()

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.

sent.c

@@ -436,6 +436,7 @@ advance(const Arg *arg)
 		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]);