SVG support extension

[edemaine]

Here are some incompatibilities importing SVG that I've bumped into. Help implementing them would be appreciated; else I will work on them as time allows.

• Paths do not support z/Z which closes the path (common in Inkscape, for example)
• Global <style> is not supported (and warned), but common with Adobe Illustrator output.
• Groups (<g>) with transforms are not supported (but transforms on rendering elements are supported). Sadly not dealt with by THREE.SVGLoader. I had to work around this in https://erikdemaine.org/fonts/maze/ (see source).
• Objects in <defs> are treated as regular objects. This is annoying because Inkscape tends to make an invisible <rect> in there, which causes a warning (because it has no stroke). Probably same issue with <symbol>.
• <use> will not expand <defs>/<symbol>s. This is probably less important than the previous item, but I've worked around it many times, so would love to have it.