Exposed Map::addCustomLayer() publicly as an undocumented, block-based API. This change avoids the terms “layer”, “initialize”, “render”, and “altitude” because they overload existing terminology in the SDK, UIKit, or MapKit. (“Layer” is qualified as “style layer”.) There are outstanding questions about ARC memory management, but this approach works in both Objective-C and Swift for pure OpenGL programming.
Added a demo to iosapp that draws a green layer below any labels in Mapbox Streets.
