code: public void pushFramebuffer(int fbuf, int width, int height) {
      assert pushedFramebuffer == -1 : "Already have a pushed framebuffer";
      pushedFramebuffer = lastFramebuffer;
      pushedWidth = curFbufWidth;
      pushedHeight = curFbufHeight;
      bindFramebuffer(fbuf, width, height);
    }
docs: Stores the metadata for the currently bound frame buffer, and binds the supplied framebuffer.
  This must be followed by a call to {@link #popFramebuffer}. Also, it is not allowed to push a
  framebuffer if a framebuffer is already pushed. Only one level of nesting is supported.