code: protected String textureFragmentShader() {
      StringBuilder str = new StringBuilder(FRAGMENT_PREAMBLE);
  
      str.append(textureUniforms());
      str.append(textureVaryings());
  
      str.append("void main(void) {\n");
  
      str.append(textureColor());
      str.append(textureTint());
      str.append(textureAlpha());
  
      str.append(
        "  gl_FragColor = textureColor;\n" +
        "}");
  
      return str.toString();
    }
docs: Returns the texture fragment shader program. Note that this program <em>must</em> preserve the
  use of the existing varying attributes. You can add new varying attributes, but you cannot
  remove or change the defaults.