[IMG_DIR] not used in pdf submodule

Issue Summary

The pdf converter submodule uses the default directory set by File system module as File system path rather than the DruTeX module user defined image directory and URL. The image URL points to this same place. Moreover I am using an absolute location outside of document root for the file system path. The constructed URL is nonsense both syntactically and functionally since the directory is not relative to document root, e.g.,if file system path is /real_file_system, pdf and tex files are written to /real_file_system/tex and the image URL becomes

The base DruTeX module properly uses the user supplied image directory and URL for images.