convert: HD support (re: #10) (the real thing)

/me learns

This PR makes the convert script work with HD Edition's resources, so people don't necessarily have to have the CD version, but can also get HD Edition from Steam and use that.

It relies on some obsolete files that still ship with HD Edition, so it may break in the future, if they clean up their builds.

Tested with a new HD Edition installation as of october 27, 2014.