Support multi-icon symbols

Motivation

I want to show a complex set of icons: ~100 basetypes with ~30 modifiers. There is no way to predefine ~3000 icons in the PNG sprite and then just pick the right one. I want to define ~130 and combine them freely. That already works by using two independent layers, but then "basetypes" and "modifiers" are on different z-indices so they interfere with each other.

Design Alternatives

Design 1: make "icon-image" accept an array (has to be implemented on other "icon-" options like "icon-offset" as well)

...
"icon-image": ["{basetype}", "{modifier}"]
...

Design 1a : don´t use an array on every single property but shift it up the to the "paint" and "layout" property.

Design 2: allow the definition of combined icons in the sprite JSON

...
"basetype":{"x":0,"y":0,"width":20,"height":40,"pixelRatio":1},
"modifier":{"x":20,"y":0,"width":20,"height":20,"pixelRatio":1},
"combined":{"icons":["basetype","modifier"], "offset":[[0,0],[0,20]]}
...

Design

Advantages Design 1: dynamic styling
Drawbacks Design 1: complex due to multiplication of icon-properties by number of icons

Advantages Design 2: combined icons would show up like normal icons in the stylesheet
Drawbacks Design 2: need for hard coded combined icons

The issue names no source files, tests, or entry points, so there is no direct code starting point. First resolve whether Design 1 or Design 2 is intended and define the behavior of combined icon properties; completion criteria are not specified.