Adds Texture:setAnisotropy( filter multiplier)  method

- Much better quality when enabled, gracefully handle non supporting (<3%) and multiplier set to > max hardware aniso supported
- added comments (notably the NPOT thing whe should check)
- commented the releaseGLobject that was emptying this._image, which was preventing user side dirty/recreation of gl object from same image when you just change the texture parameters. (ought to be much more friendly, that... osgjs user would expect setting a magfilter would work without him having to manually realase the gl object.)

Still in the hack format of modifying the prototype from there?
```javascript
Texture.ANISOTROPIC_SUPPORT_EXT = true;
```
But, perhaps it would make more sense to add that to WebGLCaps prototype ?
```javascript
WebGLCaps.ANISOTROPIC_SUPPORT_EXT = true;
```
or inside a profile or something
```javascript
WebGLCaps.profile.ANISOTROPIC_SUPPORT_EXT = true;
```