LayerRegistry class

Objects contributed by every layer of one map, grouped by kind and keyed by layer id.

Layers call set… whenever their objects change; the map flushes the merged state to the platform as a diff, once per frame. Object ids are '<layerId>/<key-or-index>', so a marker tap is routed back to the layer by its id prefix.

Layers stack like flutter_map's: in the order of UnifiedMap.children (layers under the same child in the order they were mounted). Each layer gets an integer z-index; when a layer is inserted below others, the layers whose z-index changes are synced again.

Updates of each kind reach the platform one at a time: a push waits for the previous one of its kind to complete, and the changes made meanwhile go out as a single diff. A diff is recorded as sent only once applied; a failed update is reported, sent again once right away, then with the next change. A transition only animates the markers of the layers that asked for one: the markers of the other layers go out first in an update without transition. Polylines, polygons and circles that change in the same frame as markers are sent once that marker update is applied, so a layer's shapes and markers reach the map together; shapes are never animated.