There are various changes put into this one commit, I am too lazy to split
them up. I have tested this passes CI for all desktop platforms + web.
I switched from Ubuntu to Debian, because dpkg --add-architecture
doesn't seem to work on Ubuntu, and it's necessary to obtain SDL so
cross-compilation works.
I removed osxcross because it builds llvm, which takes a long time, and
I didn't want to wait. It's broken/unused anyway, so not really a loss.
It can still be added back more carefully in future.
I fixed the web CI by installing Debian's emscripten package. It seems
to build fine, I downloaded the artifact and tested it locally with
emrun, and the fte help menu showed up. I'm not sure how to test
further than that, but I doubt anything is broken.
