Node v12 + protozero port + additional beta compositing support

[springmeyer]

This advances the node-fontnik codebase by:

• Adding node v12 support
• Porting to use protozero rather the libprotobuf (Port to protozero #143)
• Adds support for glyph compositing in pure C++, which is identical to glyph-pbf-composite more efficient

The compositing will be more efficient https://github.com/mapbox/glyph-pbf-composite because glyph-pbf-composite blocks the main event loop while compositing

We can learn from the designs of modules like https://github.com/mapbox/vtcomposite which have shown the lowest latency servers can be written on top of modules that:

• Do all their compute work like compositing in the C++ threadpool
• Zero-copy the buffers from JS into the C++ threadpool by referencing the buffers memory
• Handle gzip decompression and compression inside the threadpool (in the same trip into it)

TODO:

• Add tests by porting over tests from glyph-pbf-composite
• ticket out gzip compression in the threadpool as a followup step

/cc @mapbox/map-design-team @mapbox/maps-api @mapbox/api-gl