Placeholder areas

Nobody has claimed this yet.

_From @kkaefer on January 20, 2017 15:1_

I'm proposing to introduce placeholder areas: Annotations are currently located in a layer that is above all other layers, meaning that annotations will always cover existing features, such as labels. Similarly, when adding new layers, you'll have to explicitly name an existing layer as the beforeLayerID parameter to have the layer inserted below that layer.

Placeholder areas would be named regions within a style that can be referenced when inserting new layers, or adding annotations (i.e. every placeholder area would have its own annotation layer). An example JSON excerpt would look like this:

"layers": [
// background layer
// water layer
{
"area": "landmass"
},
// ... other layers ...
{
"area": "overlay"
},
// ... label layers ...
]

We'd have to establish some convention for style authors to follow, but it'll help the common case of a user inserting some annotations that cover up the labels.

There might be some way we could reuse Studio metadata (which has layer groups) to accomplish this as well.

/cc @lucaswoj @kronick @jfirebaugh @1ec5 @tmcw

_Copied from original issue: mapbox/mapbox-gl-style-spec#658_

The issue names no files, tests, or entry points. Start by locating the style-spec handling for layer insertion and annotation placement, then review how existing layer groups or Studio metadata are represented. Done requires an agreed placeholder-area convention, implementation across the affected paths, and tests for inserting layers and annotations into those areas.

Written by the indexing model from the issue text.