Support spaces for tags that only ever contain a single text string valu...

...e (i.e. material names, texture file names) in the OBJ converter.

I had some issues with some meshes that had spaces in their material and texture names, as the OBJ converter splits lines on all whitespaces. This change only does one split up front, handles tags for material names, texture names, groups, etc. that shouldn't be split first, then continues to split the rest of the line and handle the remaining tags.