Multiplatform xmgl

[Artefact2]

Use SDL3 for xmgl instead of GLFW/JACK, should be easier to distribute binaries for Linux (appimage) and Windows. The idea would be to give end users a simple demo of what libxm sounds like.

Maybe use github actions to automate building.

[Artefact2]

SDL has no way to query the system's audio latency.

Is there another cross-platform "all in one" API for pushing audio frames and getting an OpenGL context?