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)