Issue was caused by cycles trying to find builtin images in a main
database and in case of preview render images are not in database,
they're just referenced by shader node tree.
Now builtin images in cycles have got void* pointer to store data
needed to load builtin images.
In case ob blender session, this pointer will store pointer from
PointerRNA for image datablock and used later to construct Image
class based on this pointer.
This also saves database lookup for final render which is nice :)
Reviewed by Brecht.
452a52575c