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.)
we could make webglCAPS a singleton like ShaderProcessor https://github.com/cedricpinson/osgjs/blob/develop/sources/osgShader/ShaderProcessor.js#L16 and thus accessing from anywhere but stil 1 instance ?