Now take into account `dupli_offset` of groups.
Also fixed another issue related to objects renderability, previous code was giving
bad results due to added temp objects to generate previews (camera, lamp...).
Root of the issue was, preview generator was filling ID preview with unsigned int,
when RNA only knows of signed integers (and thus generates a python exception
when converting uint outside of int range)...
Using the brand new and much simple float pixels accessor to PreviewImage now.
Why this was working perfectly OK (it seems...) under Linux, and why error (py exception)
was so badly and misleadingly reported on Windows, remains pitch black mystery to me.
Cannot reproduce the issue here on linux, but have the feeling windows builds are much
more easily broken with referencing bad data from bpy. So here guessing direct loop
over bpy.data.xxx when you do add and remove stuff from said xxx inside the loop
is not a good idea - which seems logical in the end. :P
Not much to add, you can now clear previews from current .blend file, or a set of non-opened files.
Likewise, you can generate previews (for mat/tex, objects, groups, scenes, ...).
