graphics/libpano13: Update to 2.9.20

Submitter becomes maintainer
Upstream have confirmed unit tests to be broken so they're disabled
for now.

Changelog: https://sourceforge.net/projects/panotools/files/libpano13/libpano13-2.9.20/

PR: 258354
Approved by: tcberner
Differential Revision: https://reviews.freebsd.org/D31925