Port recent geojson-vt improvements and bugfixes

[mourner]

• fix a silly bug with getTile generating unnecessary tiles mapbox/geojson-vt@53ecc21
• depth-first tiling mapbox/geojson-vt@ec70671
• fix broken clipped square heuristics mapbox/geojson-vt@0d16d19 and mapbox/geojson-vt@cf21ce9
• return null on invalid/non-existing tiles mapbox/geojson-vt@27aea1c
• don't generate tiles on empty GeoJSON mapbox/geojson-vt@7cfb639
• make sure test coverage is up to date

cc @incanus @kkaefer

[mourner]

@incanus @kkaefer at least the first one should be ported and integrated into GL ASAP because it's a serious bug.

[incanus]

at least the first one should be ported and integrated into GL ASAP because it's a serious bug.

Flagging here that per mapbox/mapbox-gl-native#2226 we have a lot of changes that are also still not in GL — 40 commits and 97 changed files currently.

[kkaefer]

I ported all applicable commits in the order they appeared in JS and it's passing with all of the updated tests.