Harmonize thumbnail generation for rgb images

A little one to improve things for the release. Dealing with thumbnails of rgb images was not done very nicely. I stumbled today on an image which was 192x192 in the axial plane and it did not get resized to 128x128 for creating the thumbnails... And that looks ugly

This patch aims at correcting this (not the best way, in fact the shrink filter we use is flawed, it's only able to apply an integer shrinking). 

Also, for thumbnails that are smaller than 128x128, the background gets filled with black, so that overlaid icons on thumbnails work