Rename layer visible -> enabled & don't allow descendant layers to override

[bcamper]

As discussed in tangrams/tangram-docs#198:

• Renames the layer-level visible property to enabled, to prevent confusion with the draw group-level visible property, which continues to exist as-is.
  • Provides backwards compatibility for the previous visible property name: if the enabled parameter is undefined for a given layer, any value for visible will be used instead.
• Changes the Tangram JS treatment of the layer enabled (nee visible) parameter to match that of Tangram ES: once set by a layer, the parameter cannot be overridden by any descendant layers.
  • This provides a mechanism to cull large portions of the layer tree, with the guarantee that they will stay hidden. This is especially useful as more optional overlay-style layers are added to the map (bike, transit, etc.), allowing the engine to efficiently cull these when they are off.
  • For cases where it's useful for sub-layers to control visibility, the draw group-level visible property continues to provide this fine-grained control; since all draw group properties are merged, and descendant layer draw groups can override any properties defined by ancestors, this behavior continues to work as-is.

[bcamper]

@matteblair @nvkelso let's confirm this is the behavior we want? I'd like to include any such syntax changes in 0.12.0 rather than putting them off with further behavior divergence (and backwards compatibility transition period for previous visible property).

[matteblair]

👍 Looks good to me!

[bcamper]

Released in v0.12.0