Test performance improvements

This is branched from #1145.  Merge that first or just close it.

This improves test performance on my MacBook Pro from ~42 to ~33 seconds by providing an option to skip slow WebGL validation that rarely finds errors (but not rare enough to remove it completely).  This helps #857, but let's leave it open and I'll update it with more ideas.

Do we want to expose `getQueryParameters` in `Core` and also use it in viewer?  This is my implementation from glTF.  I think it's good, but I have no idea.

![image](https://f.cloud.github.com/assets/782098/1134380/3674091a-1bf0-11e3-8020-92321f1c3a66.png)

No update to CHANGES.md; this is just noise for end users.