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.)
Merely wanted to point a not intuitive API point, Not even sure that having that "filter slider" in material has any sense anyway ?
Should I make an Issue of that so that we can discuss it ?
(want to limit the scope of the PR to just anisotropy to have it handy when needed.)