Fix invocation with absolute path.

Invoking openscad in the current git version with an absolute path:
  ./openscad /home/username/foo.scad
fails. This is because the curent working directory always prepended; so
internally, it attempted to open $PWD + /home/username/foo.scad

Use platform aware QDir functionality to only prepend the absolute
current working directory prefix, if the given path is relative.