Don't change the opacity of the current image to avoid fade via black

[MatthiasMann]

https://github.com/linuxmint/pix/blob/7a033863297a2ffa62a59f94bf7369e8070e6452/extensions/slideshow/main.c#L139-L140

If I comment out these 2 lines I get a much smoother transition effect that doesn't go dark in between images - esp useful when the images in the slide show are mostly static (eg frames of a presentation).