Modularization of map layer types

[pozdnyakov]

This issue includes re-grouping and refactoring the map layers code and updating the build configuration, so that for each layer type we can enable or disable the related code at compile time. As a result, we will be able to exclude any layer type from the delivered binaries.

[pozdnyakov]

Layer modules can be disabled/enabled at the platform/ level.

[chloekraw]

@pozdnyakov @tmpsantos - I'm drafting a note for Snap here and I'd like to show a table of how much they can save on binary size by disabling a layer by referencing https://tmpsantos.com.br/binary/.

Quick question, let's say Snap were able to disable symbol layer -- would the size savings be limited to the 26.0kB from the style, or would the compiler flag also disable related functions like SymbolLayout, SymbolBucket, and SymbolInstance, resulting in additional size savings?

[pozdnyakov]

@chloekraw exculding a layer module from the layer manager removes all the related code from the binary including both style (style::<Layer type>Layer, style::<Layer type>LayerImpl, ..) and render (<Layer type>Bucket, <Layer type>Layout, Render<Layer type>Layer, so on..) parts + platform bindings.

For disabled symbol layer the size savings should be much more than 26 kB.

[chloekraw]

Thank you @pozdnyakov. Two quick points of clarification:

1. In this doc, expressions and annotations are included in the size estimation table. I see that we have a compile flag for annotations (LayerManager::annotationsEnabled); is there one for expressions? Is it possible for a developer to disable expressions today?

2. Could you confirm whether LayerManager is accessible on the platform side so that developers using our releases can also take advantage of size savings? My current impression is no -- only developers building Mapbox from source can add/remove layers from LayerManager and set the annotations compile flag.

[pozdnyakov]

1. Unfortunately no, expressions code is spread in our code base. It'll require a lot of work to encapsulate it in a module.
2. LayerManager is configurable at build time, e.g. with MBGL_LAYER_.. compile-time flags. It is necessary to build from source to take its advantages.
   The possible way to enable distribution size management in run time is to build every layer module into a separate dynamic library, then instead of a single core library we will have a core library + layer plugins, that could be excluded from the package.