[ios] Add iOS bindings and example for missing icons event.

[jmkiley]

Adds iOS bindings for #14253.

cc @julianrex @ansis

[fabian-guerra]

You have to assign the returned image to a variable. Then use the returned image name to load the resource and pass this back to the renderer.

[fabian-guerra]

Something along the lines of:
-(NSString *)mapView:(MGLMapView *)mapView didFailLoadingImage:(NSString *)missingImage;

[julianrex]

...didFailToLoadImage: ?

[jmkiley]

I started to implement this, but I was not able to finish porting the example. I may have misunderstood what was supposed to happen, sorry!

[ansis]

This looks good to me but I think this should get another review from someone on iOS because I don't understand that part well. This is an "approve" from my side.

If possible, it might be good to expand this test to cover a case where an icon is missing but you don't want to provide it. I've added another couple layers to the style json to test this case:

Expand to see new style json

```json { "version": 8, "name": "Mapbox Streets", "sprite": "mapbox://sprites/mapbox/streets-v8", "glyphs": "mapbox://fonts/mapbox/{fontstack}/{range}.pbf", "sources": { "point": { "type": "geojson", "data": { "type": "Feature", "properties": {}, "geometry": { "type": "Point", "coordinates": [0, 0] } } }, "skip-point": { "type": "geojson", "data": { "type": "Feature", "properties": {}, "geometry": { "type": "Point", "coordinates": [0, 40] } } } }, "layers": [{ "id": "bg", "type": "background", "paint": { "background-color": "#f00" } }, { "id": "point", "type": "circle", "source": "point", "paint": { "circle-radius": 100 } }, { "id": "icon", "type": "symbol", "source": "point", "layout": { "icon-image": "missing-icon" } }, { "id": "show-skip-point", "type": "circle", "source": "skip-point", "paint": { "circle-radius": 100, "circle-color": "#fff" } }, { "id": "skip-icon", "type": "symbol", "source": "skip-point", "layout": { "icon-image": "skip-this-missing-icon" } }] } ```

My code for this in the glfw app looks like this:

    if (id != "skip-this-missing-icon") {
        map->getStyle().addImage(makeImage(id, 22, 22, 1));
    }

It should look something like this (no icon above the white circle):

[julianrex]

iOS parts LGTM.
@ansis for whether the callback to GL is correct.

[fabian-guerra]

This is how it's rendered on iOS with the substitute image:
cc @ansis @julianrex