Fix segfault on close content on Windows

[bslenul]

I_ShutdownGraphics() is called during CallTerminateFunctions() so delete "screen" here, this prevents a crash on close content on Windows (and I double checked on my Linux VM, it doesn't break anything and it still closes fine there).

Also removed the "'xxx' is freed outside the GC process." warnings.

edit: Unrelated but force pushed some removals for repeated values in the "SLIDER_OPTIONS" macro that I just spotted.

Closes #91