Fix set/unset range thumbnails

Open Krifa75 requested to merge Ordissimo/eog:range-thumbnails into master

On a very slow machine, in the gallery when I scroll horizontal, sometimes the previous thumbnail is half visible and so the method eog_thumb_view_clear/add_range is called repeatedly and load/unload this range.

The commit 94e2029e fix this problem by only unloading the second previous thumbnail. That mean :

we unload this thumbnail Still loaded The first visible thumbnail
Image 1 Image 2 Image 3

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