Fix T49357: OBJ importer cannot deal with texture names with spaces.

OBJ/MTL does not support spaces in filenames, but... users keep using them - and other exporters
keep failing at being smart for the user here (like, replacing spaces with underscore etc.)...

So now, when last part of a texture line is not giving any valid found image, we try backward
adding more pieces to image filename until we find some existing file (hopefully).

But again: please do not use spaces in filenames for OBJ, **THIS IS NOT SUPPORTED**!