Terrain optimizations

[pjcozzi]

From our Santa experience:

• Combine children to minimize requests
• Land-only shader
• Optimize water shader

@kring which of these are reasonable before merging into master? There's a chance I can support the last one.

[kring]

I think these can all wait until after the merge to master.

[pjcozzi]

But they're all soooooooooooo cool!

OK, you are right.

[mramato]

Are all of these items on the terrain roadmap? Can we close this?

[pjcozzi]

@kring are these all accounted for in #526? If so, let's close this as duplicate.

[kring]

I'm not sure what "optimize water shader" entails exactly, but the others are now included on the roadmap.

[pjcozzi]

Sounds like something I said but given it's not concrete, we'll close this.