[core] Polymorphic bucket creation

[jfirebaugh]

Fifth in a series of PRs that builds towards a full-fledged programmatic style API. This is a big chunk of work, but I'm trying to break it up step by step and not create a long-lived branch with a lot of changes.

This episode moves further towards being able to remove the generic ClassProperties in favor of concrete paint and layout classes like those found in style_properties.hpp. The way that I'm going to do that is to continue refactoring until all usage of ClassProperties is in layer subclasses, and then substitute the concrete per-layer replacements.

This PR eliminates StyleBucket and moves bucket creation to layer subclasses. Along the way I believe to have fixed #2820, by restoring the previous use of a partialParse boolean.

👀 @kkaefer @brunoabinader

[kkaefer]

restoring the previous use of a partialParse boolean.

This means that we don't parse any symbol buckets that are after one that is missing. We could still parse them, but not place them.

[jfirebaugh]

I think it's set up so that they are parsed, but not placed: SymbolLayer::createBucket calls parseFeatures unconditionally, but addFeatures only if partialParse is false:

https://github.com/mapbox/mapbox-gl-native/pull/2873/files#diff-511e12d83fd87b94fa15dcdbf0d27bbcR159

Then TileWorker adds the bucket to the pending set if partialParse is true and it's a symbol bucket:

mapbox-gl-native/src/mbgl/map/tile_worker.cpp

Line 140 in ca25594

if (layer.type == StyleLayerType::Symbol && partialParse) {

Let me know if that looks correct.

[kkaefer]

There are actually three steps:

• parseFeatures: determines the text we need to display by replacing tokens and applying text transformations, and the glyphs we need
• addFeatures: determines the potential position of labels
• placeFeatures: actually places them and adds them to a buffer

placeFeatures is currently called as the last step in addFeatures, but there's no good reason to; we could separate that out and call the addFeatures without placing as soon as we have the symbols. However, that means we'd have to introduce a third step beyond pending.