mostly remove the svg onClick hack

- chrome no longer throws a security warning when trying to read pixel
  data on a canvas after drawing *any* svg; only svgs with a &lt;foreignObject&gt;
  tag in them
- Standard Safari is still an old enough WebKit that it has this problem,
  but WebKit-nightly (SVN r173347) does not.
- instead of the blanket check, just try it and catch errors