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.)
Let me chime in :). I'm not in favour of singletons, but I understand some cases where is the easier way to solve situations where "single instances" (instead of singletons) are not enough. 

But if we have to "have" a singleton I rather to have it in a Registry pattern, where we can clean all the singletons easily. Also it seems to me more design-robust and the user "knows" or at least guess that it is a singleton. 

Thougths?