[CODE] Some code I wrote

I had previously written some code (PR #3589) for anisotropic texture filtering in Citra. However, since mipmap wasn’t implemented, it was put on hold.

Now that wwylele has implemented mipmaps (not merged yet), I think that my code can be reused. Sorrily, I’m unable to contribute to Citra for atleast a year or so :cry:. So, if someone’s looking for a place to start with video_core, they can pick this up if they want.

Thanks in advance for picking it up!