Fix obj .mtl file loading

Fix obj .mtl file loading

Fix the obj file loader by adding a new method which allows
a name to be read considering the space in the middle between two
words and use that for parsing the "mtlib" line in the .obj file
parsing method.

Before, the method used in the obj parsing function would have
returned the string "mtlib NAME_OF_MTL" instead of "mtlib" only,
which resulted in the .mtl file being never parsed.