Buggy ring classification on GeoJSON multipolygons

[mourner]

mapbox-gl-js version: 0.38

When displaying complex multipolygons with holes, there seems to be a bug happening somewhere around the ring classification routine that causes severe striping artifacts when triangulating:

If I simply filter out all small rings (length < 20), the artifacts are gone. I'm not yet sure the root is ring classification, but it intuitively looks like the culprit. Needs to be investigated further.

@peterqliu if this is indeed a separate bug from earcutting polygons with small self-intersections, perhaps we could fix some of the artifacts seen on the isochrone demo.

[peterqliu]

I'm a bit out of my depth here, but the rendering bugs on the isochrones demo aren't stripes, but failures to fully flex inward, on concave portions of a polygon. Here's the same thing at two slightly different zooms:

Bad:

Good:

Approximate location for ^: https://www.mapbox.com/bites/00156/#9.21/52.5851/13.3365

[mourner]

I no longer have a test case from the original report, so closing in favor of tracking issues in #7023.