* Eliminates the need for EnableImmutableFromThis
* Eliminates the dependency of {Source,Layer}::Impl on corresponding Render class (circular dependency)
ported from -js: eb6c6596c6a7a61363d30356674e0002153b1d19
`altitude` was a terribly-named variable that was used to indirectly
control the fov. This should eliminate some confusion.
`altitude` was equivalent to `cameraToCenterDistance / height`
This converts the style property classes (CirclePaintProperties and so on) to the same tuple-based approach as gl::Attribute and gl::Uniform. The approach is outlined in https://github.com/mapbox/cpp/blob/master/C%2B%2B%20Structural%20Metaprogramming.md.
The main advantage of this approach is it allows writing algorithms that work on sets of style properties, without resorting to code generation or manually repetitive code. This lets us iterate on approaches to data-driven properties more easily.
Another advantage is that the cascading, unevaluated, and evaluated states of a set of properties exist as independent structures, instead of individual properties holding their own state. This is a more functional approach that makes data flow clearer and reduces state.
When a source or layer is removed transfer ownership back to the
caller so it can (optionally) take it. Preserve the behavior that
removing a CustomLayer triggers deinitialization. Deinitialize all
custom layers when a style is destroyed in case those layers are not
explicitly removed.
