I have no idea why but Ubuntu doesn’t allow to configure the screensavers it comes with. Not those screensavers were not configurable, but in contrast to xscreensaver-demo, Ubuntu itself does not offer access to their visual config 🙁
What I wanted to do is stop GLMatrix from rotating the matrix, which would have been
-no-rotate on the command line. Editing
/usr/share/applications/screensavers finally helped:
The modified file looks like this:
Name=GLMatrix (without rotation)
Comment=Draws 3D dropping characters similar to what is seen in the title sequence of “The Matrix”. [..] Written by Jamie Zawinski.
Exec=glmatrix -root -no-rotate