Extra display layers

**DO NOT MERGE THIS YET I'M CREATING THE PULL REQUEST SO WE CAN TALK ABOUT WHAT NEEDS TO BE DONE BEFORE WE MERGE.**

Here's extra layers! It works in webgl and has a reasonable interface (the overlay options panel is tabbed). There are some remaining issues that I think need to be worked out, and probably a lot of crap that should be cleaned up before this is merged.

Styling is probably the biggest issue -- different display layers should be styled differently, and Mark created configuration options for separate layers that should work, but right now webgl doesn't respect any of that.

What else? What needs to happen?