Add a setting to force a specific anisotropic filtering level

[Calinou]

Texture sampling quality can be improved by forcing anisotropic filtering to 16x (or perhaps 8x on integrated graphics), but Xenia currently doesn't expose an option for it in its configuration file.

I've forced anisotropic filtering via the NVIDIA Control Panel and it seems to be working fine on my end. Not every game might behave well with this option, but I'd wager a large part of them do.