As it turns out, GTK 4 programs start up *a lot* faster by setting the
environment variable `GSK_RENDERER=cairo`.

Even the most simple GTK 4 program that shows nothing but a blank window
spends a) tons of times populating `~/.cache/mesa_shader_cache` on the
first run, b) considerably less but still lots of time initializing DRI
on consecutive runs. Apparently, both these things can be avoided by
setting that variable.

That's good news for the GTK 4 port of [xiate], I think. With that
variable set, terminal windows now start up reasonably fast. Of course,
that means that the terminal can't use OpenGL during runtime, but why
would we need that? (I never understood that about Alacritty or kitty.
Unless you're doing benchmarks, this doesn't matter at all.)

So ... I guess I'll do some more work on that GTK 4 port now.

[xiate]: https://uninformativ.de/git/xiate