With this I can finally follow xlink:href and url(#...) sort of
references within the SVG doc and subset the elements accordingly so
that only those that are reachable from the initial set of glyph
elements are kept.
support for namespaces and xpath is insufficient in built-in ElementTree; supporting both lxml and ElementTree is too complicated, let's simply require lxml to be able to subset SVG for now
this drops svg document records when they no longer intersect the subset. It keeps them in their entirety (for now) when they still intersect the subset, only renaming all the id='glyphXXX' to point to the new glyph indices after subsetting. Unused, unreferenced elements are not pruned yet.
