Fullscreen doesn't work

[Seth Hetu]

Pressing Enter does not make the app fullscreen. On Linux, this is partly due to a faulty fullscreen implementation. On Windows, it's because we're missing a function in GM5Compat. Should be easy to fix on Windows at least.

[Seth Hetu]

Fixed on Linux. Windows sorta works, but there's a weird offset. OS-X works in principle but "real fullscreen" is really needed.

So... 33% done.

[Seth Hetu]

Fixed in OS-X; I'm opening a separate issue for Windows, since it's not exactly the same thing (and might not even be noticeable for the default fullscreen option).