Upgrading to clang++ 7.x

It would be nice to be able to use the latest version of clang++. However we'll hit:

```
In file included from ../src/vtshaver.cpp:1:
In file included from ../src/filters.hpp:4:
In file included from /Users/danespringmeyer/projects/vtshaver/mason_packages/.link/include/mbgl/style/filter.hpp:4:
In file included from /Users/danespringmeyer/projects/vtshaver/mason_packages/.link/include/mbgl/util/feature.hpp:3:
In file included from /Users/danespringmeyer/projects/vtshaver/mason_packages/.link/include/mbgl/util/optional.hpp:3:
/Users/danespringmeyer/projects/vtshaver/mason_packages/osx-x86_64/clang++/7.0.0/include/c++/v1/experimental/optional:11:2: error: " has been removed. Use
instead."
#error " has been removed. Use instead."
```

So, we'll need to upgrade mbgl to avoid this (#9) to a version that includes https://github.com/mapbox/mapbox-gl-native/pull/13049

Start with src/vtshaver.cpp and src/filters.hpp, then inspect the mbgl dependency version and the clang++ 7.0.0 error for experimental/optional. Update mbgl to a version including mapbox/mapbox-gl-native#13049, then verify that the project builds with clang++ 7.x.

Written by the indexing model from the issue text.