Implements paintFormXObject in svg.js

This PR partly addresses #4985 and makes the image on page 6 of the Tracemonkey paper render properly.

The addition of `this.cgrp.appendChild(this.tgrp);` is necessary to make the text elements appear after clipping in `paintFormXObjectBegin`, which actually resolves a bug in the current master and makes the text in the images on pages 10, 11 and 13 also render now.

The only issue left is the image on page 2 of the Tracemonkey paper, but it doesn't appear to be caused by this PR, but it rather appears to be another bug in the current master. @pramodhkp Could you take a look what's causing the issue on page 2? If it's indeed a bug in the current master, it can be fixed in a separate PR.