support polygon annotation stroke widths

[incanus]

Blocked upstream by mapbox/mapbox-gl-style-spec#223. Currently we just have fill outlines, which are very faint and with non-adjustable stroke width.

Workaround for now is adding a closed polygon annotation for fill, then a closed polyline (or two) with stroke atop, such as in https://github.com/incanus/WWDSee/blob/35c7c19922d7d339d5033dca99085232a5384b4f/WWDSee/ViewController.swift#L235-L244.

[tkrajacic]

What is the plan on styling those overlays in general?
Wouldn't it be great to be able to style them the same way as the vector tile sources in Mapbox Studio (using a stylesheet)? (Answer: Yes it would)
I am not sure if this would work with the current rendering pipeline.

This would be awesome for example: I am thinking about a fat semi-transparent stroke on the outline, clip it to the polygon to have basically an 'inside' stroke, then add another thin opaque stroke. (Illustrator mockup below)

With Apple's MapKit you can draw pretty much anything in your overlays (using CoreGraphics) iirc.

Just thinking out loud here. Not sure if this is what you have in mind for overlays.

[ljbade]

This ticket is superseeded by #837 which will allow the full range of style properties.