Better marker styling for Mapillary panoramic images

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Panoramic images are cool! They render in the viewer with yellowish arrows and you can drag the viewer around 360 degrees:

But we don't handle them correctly. I discovered that when we call the Mapillary API, we get the same key multiple times with different camera angles:

> feature.properties.key
"vdjJHj_NLRqcYa08vElpZQ"
> feature.properties.ca
187.626653150049
//... some time later...
> feature.properties.key
"vdjJHj_NLRqcYa08vElpZQ"
> feature.properties.ca
203.359039340867

We are expecting this key to be normal unique key, so d3 will only show one of them as a normal marker. We should instead detect this condition in the loaded callback, set a flag that it is a panoramic image, and style the panoramic markers differently, not using ca (camera angle).

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The latest Mapillary API returns a pano flag if the image is panoramic, so I'm using this to render the viewfield as a circle: