Looks good to me, thanks!
How about also adding a unit test for this? :) Since it's a text-based format, comparing some small image against the expected result probably would probably work

Sure, here's an attempt, but I don't know if it works, because my Ubuntu enviroment can't find GLib (and apparently neither can the GitHub checks), and I haven't had the time to find out why.

In my opinion, though, if properly testing this becomes too daunting, the testing could be left for later.

I haven't looked in depth, but you may need to call the Module.Initialize() functions somewhere in Pinta.Core.Tests at startup, similar to https://github.com/PintaProject/Pinta/blob/master/tests/Pinta.Effects.Tests/Utilities.cs#L11 in the effects tests. Right now I only see the text engine test doing this for Pango

Apparently, that's exactly what was needed. The tests are now running and passing as expected

Looks good, thanks!