code: public void setSpecularColor(float r, float g, float b, float a) {
          setVec4("specular_color", r, g, b, a);
      }
docs: Set the {@code specular_color} uniform for lighting.
  
  By convention, GVRF shaders can use a {@code vec4} uniform named
  {@code specular_color}. With the {@linkplain GVRShaderType.Texture
  hader,} this allows you to add an overlay specular light color on
  top of the texture. Values are between {@code 0.0f} and {@code 1.0f},
  inclusive.
  
  @param r
  Red
  @param g
  Green
  @param b
  Blue
  @param a
  Alpha