x11/xscreensaver: Update 6.12 => 6.13

Optimize post-{patch,install} stages to eliminate unnecessary process
spawn and improve readability.
Make anti-NSFW patch even less suspicious. Everybody loves cats!

Changelog:

• Added a number of historical full chess games to endgame.
• Rewrote glslideshow to add more image-transition effects.
• Hacks that display images and their filenames will display their titles intead, if the image originated in an RSS feed (or has the “user.dublincore.title” xattr).
• X11: Wayland fixes.
• X11: Some hacks were calling glXSwapBuffers before the first call to glXMakeCurrent; allow it.
• X11: Added configuration files to allow building of a non-insane .deb installer package.

https://www.jwz.org/xscreensaver/changelog.html

PR: 292582