code: public String generateSignature(HashMap<String, Integer> defined, GVRLight[] lightlist)
      {
          return getClass().getSimpleName();
      }
docs: Create a unique signature for this shader.
  The signature for simple shaders is just the class name.
  For the more complex shaders generated by GVRShaderTemplate
  the signature includes information about the vertex attributes,
  uniforms, textures and lights used by the shader variant.
  
  @param defined
  names to be defined for this shader
  @return string signature for shader
  @see GVRShaderTemplate