[gthumb-list] Enlarging small photos in slideshow

[Greg Mansfield <G Mansfield computer org>]

During a slideshow, images that are smaller the the screen are zoomed up to fit the 
screen.  The image quality suffers.

I suggest changing slideshow image zoom to: GTH_ZOOM_CHANGE_FIT_SIZE_IF_LARGER and 
using GTH_ZOOM_QUALITY_HIGH.

Thanks,
Greg Mansfield