Add support anisotropic filtering and for limiting the maximum number of mipmaps

[BonsaiDen]

Limiting the number of mip maps can be useful when dealing with large textures or ones that are updated every frame (e.g. shadow maps) to avoid performance overhead.

Anisotropic filtering requires an extension, however that is pretty much available everywhere and is also supported in WebGL / GL ES contexts.

See also the corresponding changes in three-d-asset here: asny/three-d-asset#41

[asny]

I did something a bit different in aaeb43a, but basically the same functionality. Do you have any feedback to those changes?
I also updated the texture example in cb508f6 which now shows the different settings.
Also, see #510