Update style spec, gen new features, remove obsolete zIndex

[tobrun]

This PR updates to the latest style spec and a recent pre release build of the Mapbox maps SDK.

Todo:

• move towards using a git submodule of gl-js instead of copy/pasting this file into this repo
• remove old zIndex setup in favor of SymbolSortKey

[tobrun]

Note this PR also updates the Maps SDK to v7.4.0

[tobrun]

Been running some benchmarks for animating symbols (the animatied cars driving on a map example):

no animation

Amount frames 296
Average FPS 55.82793574431573

SymbolLayer - animated

Amount frames 374
Average FPS 41.81102010196368

SymbolManager - before

Amount frames 209
Average FPS 25.355876562292273

SymbolManager - without zIndex (this PR)

Amount frames 233
Average FPS 28.16800860352976

Notes:

• the low lever symbol layer is better for performance. If scale and performance are of importance use the low level symbol layer API instead
• this improves performance a bit but was initially hoping on a larger impact.

[LukasPaczos]

I see nitpick errors, but regardless of that, try rebasing on master if you'll see unit tests failing (#923 (comment)).

[tobrun]

I see nitpick errors

that was because git submodules weren't correctly intialised on CI. This PR is ready for review.

[LukasPaczos]

🚢