graphics/swappy: update to 1.7.1

While here:
- switch to use upstream release tarball
- split PLIST_FILES into pkg-plist
- move post-patch target to patch file since the change is about
  static content
- sort variable order to make portclippy happy

Changelog: https://github.com/jtheoof/swappy/blob/v1.7.1/CHANGELOG.md

Reported by:	GitHub (watch releases)