Still not implemented:
1) Blending using linear RGB
2) Doesn't work if a shape has a filter stack
3) Shapes are leaking... but it should be fixed on the later
   stages of implementing SVG support. Right now shape ownership
   rules are not quite clear to me.
4) Clip mask will not be found if placed into <def> category. *But*
   at the moment the <def> feature is implemented incorrectly. The
   inheritance of the 'def' elements should come from 'def', but not
   from the place of instantiation. So this part should be refactored
   anyway.