MGLMultiPolygon doesn't support coordinate property

[aayers-agrible]

Platform:
iOS
Mapbox SDK version:
3.3.6

Steps to trigger behavior

1. Create a MGLMultiPolygon object (I parse them from a geojson source)
2. Request the MGLMultiPolygon object's 'coordinate' property
3. Get "MGLShape is an abstract class error"

Expected behavior

The centroid of the MultiPoly would be returned from the coordinate property

Actual behavior

I get an 'MGLShape is an abstract class' error.

[1ec5]

There are two issues here:

• -[MGLMultiPolygon coordinate] should be implemented in order to avoid the exception in MGLShape. For now, we can have it return the first coordinate, as MGLPolyline and MGLPolygon do.
• -[MGLPolyline coordinate], -[MGLPolygon coordinate], and -[MGLMultiPolygon coordinate] should return the centroid. This is a larger task that would require Polylabel.

/cc @mourner

[mourner]

@1ec5 wasn't Polylabel already integrated recently by @jfirebaugh?

[1ec5]

Ah, I missed that it got integrated (for label placement) in #7465. We'll have to figure out a way to make it accessible to SDK code as well.