Define a Circle based on Physical Units

[riastrad]

The Android Maps team just posted a proof of concept for using TurfTransformation#circle() to
create a FillLayer that defines a Circle in physical units (i.e. "meters") rather than in pixels. The original PR is here: mapbox/mapbox-gl-native#14429

This would be a great addition to the demo app.

cc/ @langsmith @osana

[langsmith]

A good addition for this repo's demo app example would be to show the radius distance in degrees, radians, miles, or kilometers. As the slider’s used in the gl-native test app example, I don’t know if the radius is equivalent to 1 mile or 50 kilometers. A toggle between the various distance measurements could be cool too for this repo's example.

[riastrad]

A toggle between the various distance measurements could be cool too for this repo's example.

Really like this idea, @langsmith! I'm also thinking it will look best if we put it over an urban area so you have a somewhat clearer idea of the size reference (because buildings).

[riastrad]

Noting that the originally linked PR omitted some code. Finalized version of the implementation can be viewed here: mapbox/mapbox-gl-native#14437