One can now mark a texture param in a material definition as linear by using -Linear at the end of the parameter declaration (case insensitive).
An Image bound to a material texture param in a linear color space will have its ColorSpace set to linear when bound to the material by mat.setTexture("name", texture);
Added the -LINEAR flag to all texture param that needed it in stock materials (lighting.j3md, terrainLighting.j3md, different Filter materials).
