Store a scaled version of each image to make redraws faster

This is important for fading, see bug #15. I decided to use another
image surface for this, because during fades, there are 2 images which
must both be drawn: The old one AND the new one. Now
last_visible_image_surface stores the old image, and the new variable
stores the scaled copy of the new one.
Interrupt slideshows until the next image has been loaded

Previously the slideshow would move to the next image after n seconds,
regarless of any circumstances (user moved to another image, image not
loaded yet...)