Option to limit orbit pitch (don't orbit below the scene) #332

We embed the viewer with createViewer and want orbit mode to stop at
the horizon, so a visitor can't orbit under the scene and see the splat
from below.

The range is fixed today: OrbitController sets
controller.pitchRange = new Vec2(-90, 90), and the controllers aren't
reachable from the handle, so an embedder can't change it.

Could orbit's pitch range be an option, for example
createViewer({ orbit: { pitchRange: [-90, 0] } }) or a field in
settings.json? We're carrying a one-line patch until then.

Related: #77.