JSON keys in our style specification don't have a defined order. This change sorts them alphabetically so that we can rely on the order remaining them same across code generation runs.
`LayerManager` is now responsible for `RenderLayer` instances creation,
so that there is a single entry point for creating of objects, which
correspond to a certain layer type.
The `LayerType type` field is dropped from `Layer::Impl`.
The `LayerTypeInfo` contains static meta data about certain layer type.
Each layer module should have a single immutable `LayerTypeInfo` instance
for the represented layer type. Both `LayerImpl` and `LayerFactory` from the
module always refer to the same `LayerTypeInfo` instance, so address of this
instance can be used as a layer module Id during the process life time.
This patch introduces the initial implementation of
- A `LayerFactory` abstract class that creates `style::Layer` instances of
a certain layer type (line, hillshade, round, ..)
- A singleton `LayerManager` class, which is responsible for initializing
the `LayerFactory` instances and forwarding the `create()` calls to the
corresponding factory.
The `style::Layer` class now exposes all the properties contained at `style::LayerImpl`.
This allowed to drop `style::Layer::accept()` method usage, avoid the repeated generated code
and thus save some binary size.
This patch is a part of the layers modularization effort.
Based on patch from @lbud (Lauren Budorick).
Give `HeatmapColorPropertyValue` a more generic name,
since the same value type will be used for both `heatmap-color`
and `line-gradient` properties.
