code: public Surface clear (float red, float green, float blue, float alpha) {
      batch.gl.glClearColor(red, green, blue, alpha);
      batch.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
      return this;
    }
docs: Clears the entire surface to the specified color.
  The channels are values in the range {@code [0,1]}.