name w/o path: assume textures are in the folder, where model file is
absolute path or with ..: try to cut at "/models/" or "/textures/" (as if leading to game folders); do previous case, if not
strip extensions and turn slashes to forward ones anytime
* picomodel::obj: fix diffuse map paths loading from .mtl
There's a bug in picolib when parsing the first group in an .obj file.
The library tries to set the surface name to a surface pointer
that is still NULL.
This gets silently ignored but the problem reappears later
in a warning message when the material or a face is parsed.
