[Gtkradiant] Patch for TODO Textures: add anisotropic filtering

[johnfitz at u.washington.edu]

I did some tests in an app that supports anisotropy (fitzquake) and it seems that it provides a sampling improvement no matter what minification filter you use - even those that aren't mipmapped.

However, for simplicity of interface, it makes sense to simply tack on "anisotropic" as a new setting after "trilinear", and have it mean:

mag filter: GL_LINEAR
min filter: GL_LINEAR_MIPMAP_LINEAR
anisotropy: whatever value is reported for GL_MAX_TEXTURE_MAX_ANISOTROPY_EXT

That way there is a simple sequence of lowest-to-highest quality options presented to the user.

John

P.S. I'm confused by the existence of both "linear" and "bilinear" options, as the terms mean the same thing AFAIK.