Fix rendering artifacts with round line joins and semi-transparent or gap-width lines

[lbud]

test case: https://www.mapbox.com/bites/00059/#15.1/36.9060/-76.0800

[lbud]

(follow-up from #681)
More clearly:

[ljbade]

I have noticed this in native too. Came up recently when my line dashes were bugging out but still see it on other transparent lines.

[ansis]

We now skip adding round joins for shallow joins where miters are indistinguishable from round joins. This removes most of the dots within a segment. There are still overlapping dots where different segments meet:

all roads in the image have "line-join": "round"

We could make the tessellation slightly more complicated to get rid of all overlap within a segment, but I don't think this matters if we don't also get rid of overlap across different segments.

For overlap between different segments there are two ways this could be done:

• draw into a separate texture first, then composite it. We tried this. It's too slow.
• prevent double drawing with the depth buffer. With this approach lines would be drawn twice. Once for the solid part and once for the antialiased/blurred edge. There would still be tiny 1px artifacts where the antialiased/blurred edges for different features overlap, but I don't think we can avoid that without having a huge performance cost. Something similar could be implemented for polygons.

[mourner]

It seems like currently only sharp corners have this problem:

[mourner]

Closing as we addressed this well enough it seems.