https://github.com/mapbox/mapbox-gl-js/pull/1359
Round line joins used to be drawn by adding a semicircle cap to the end
of each segment. This looked fine for opaque lines not for
semi-transparent lines.
This changes the triangulation so that round line joins don't overlap
with segments. The gap between segments is filled with small triangles
that look like pie slices. The edge of the round linejoin is made up of
many short straight lines that look round at the sizes we draw lines.
Since sharp angles are infrequent, this does not significantly affect
the total number of triangles created.
Joins for angles that are really sharp are still drawn with overlap.
888ae5ebdc