Implement tri-linear texture sampling in the texture API

[Dawoodoz]

Tri-linear texture sampling would allow raising the sampling quality on 3D rendering using yet another template argument. It is done by sampling two mip levels when close to a transition and fading linearly between them to make surfaces look more natural. If lightmaps and diffuse maps are mixed in advance using dynamic lightmaps, it will not be too expensive on cheap laptops.

Anisotropic filtering would however be a bit pointless without dedicated hardware for anti-aliasing, so high quality rendering can just render to a higher resolution with super sampling, or use random offsets with undersampling of sharper MIP layers and motion blur as a temporal filter.

[Dawoodoz]

It might be possible to provide samplers for individual textures as function pointers with neglectable overhead, by letting the calls operate on upper and lower rows of pixels in stack memory. The extra cycles spent on L1 data cache access would be won back by reduced trashing of instruction cache and not running out of SIMD registers, when one function at a time does something simpler. By splitting it up, any texture slot can choose between nearest-neighbor, bi-linear and tri-linear texture sampling. Anisotropic sampling could also be added, because super-sampling the whole scene might not be enough.