Blender Git Loki

Fix some issues with image previews.

* When overlaying icon, we have to also ensure background pict is always the same size
(else you get usgly icon size changes). Still not perfect though, suspect some more issues
in filrdraw code, probably should systematiacally run the 'ensure size' code during thumbnail
generation?
* Sharing of img ImBuf between thumbnail job and main thread could still happen,
leading to random crashes.