Relative path to input file is doubled on output file

[glenn-horton-smith]

If a relative path is used for the input file name, the output directory has that relative path effectively doubled.
E.g., if the input filename is ../test.txt, the output is written to ../../test.zip, relative to the current directory in which text2qti was run.
The problem occurs because main() changes into the directory of the input file before running, but then also prepends the directory part of the input file to the output file.
This has no effect on absolute paths, but doubles the relative path.
Will make a PR shortly with the fix. (Correctly, this time.)