`LayerManager` is now responsible for `RenderLayer` instances creation,
so that there is a single entry point for creating of objects, which
correspond to a certain layer type.
The `LayerType type` field is dropped from `Layer::Impl`.
* [core] geometry.hpp 0.8.0
* geojsonvt @ 6.0.0
* Update platform deps, build scripts
* Perf optimizations/cleanup
* Rebase in geometry@080
* D.R.Y. etc
* Ensure fill annotation geometries have closed rings.
* Optimizations
* Update to geojsonvt @ 6.1.0 for clean handoff between geojson parsing and geojsonvt
* Apply close multi/poly geoms for line annotations as well
* Layer source ID is immutable; must be provided to the constructor
* Layer source layer is mutable
* Layers with GeoJSON sources do not have a source layer
While here, make Layer::copy impl-private.
