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

[MatthiasMann]

pix/extensions/slideshow/main.c

Lines 139 to 140 in 7a03386

	if (self->current_image != NULL)
	clutter_actor_set_opacity (self->current_image, (int) VALUE_AT_PROGRESS (255.0, 1.0 - progress));

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).