Add support for quoted filenames in mtllib entries

[Sakai-san]

• Handle quoted filenames (single or double quotes) in mtllib statements
• Support filenames with spaces when properly quoted
• Maintain backward compatibility with unquoted filenames

Examples:

mtllib "wall.mtl => ['mtllib', '"wall.mtl']

mtllib 'w a l l.mtl' => ['mtllib', 'w a l l.mtl']

mtllib "wa l l.mtl" => ['mtllib', 'wa l l.mtl']

mtllib wall.mtl' => ['mtllib', "wall.mtl'"]

mtllib wa l l.mtl => ['mtllib', 'wa l l.mtl']