First important milestone

This is a very small step, but an important one: I managed to get a simple SDL2 example program in rust to cross compile for arm64 and actually run it on an RG351V.

The Cross crate for rust is a bit ridiculous but it got the job done (of course not with me not creating a custom Dockerfile to add SDL2 to the build image).

Even though the source says nothing about full screen, on the 351V, the app opened up in full screen (probably a feature of their “window manager”) and did it's thing (a simple color cycle).

Next steps will be to figure out how to detect the screen size and scale the window respectively. I will figure out how to scale the UI later. And then add support for the controller surface so that I can somehow quit the app as well (as the ESC key is notably absent from retro consoles.