bug#58081: no video in celluloid

[Csepp]

There is no video output from Celluloid.  No idea how to get debug
output from it, it's not printing anything on standard out and the
built-in debug shell doesn't work since that relies on video output I
guess.

Okay so it does work if I run it with LIBGL_ALWAYS_SOFTWARE=1, so I
guess that's a step forward.

But Celluloid is supposed to just be a GUI for MPV and MPV works
perfectly fine on my system, so it shouldn't be a driver issue.