TODO
  * SVGUseElement -> fix GetBBox, GetResultBBox if no width/height specified
  * SVGLocatable::GetChildrenBBox -> don't use defs-tag by calculation
  * Render: don't delete shadow tree aftre drawing of SVGUseElement
  * SVG*Elements -> extend only from one abstract class and
    multiply interfaces; set methods -> set status of element
    to 'changed': render -> canvas items must be recreated only
    for elments with status 'changed'.
  * rewrite parse_dtd.py using xmlto library to parse svg11.dtd
  * implement animated style attributes
  * fix SVGAnimatedPathData and SVGAnimatedPoints
  * libart backend: optimize DrawCanvasImage -> too slow