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.)
That's what I spotted, either the choice was made but whitout comment we do not know or the code is just from a time where implementors didn't enalbe the arb npot ext on their side