Rendering curved lines

Nobody has claimed this yet.

## Motivation

Both the GeoJSON and vector tile specifications encode line geometry as discrete points, with straight line segments connecting them. This is a great, lightweight format to store and render geometry, but makes it difficult to render curves, either when

1) we want actual curves, or
2) when we want to draw complex, organic shapes with minimal number of vertices. Curves would allow us to approximate the original line by applying smoothening on the render side.

https://github.com/mapbox/mapbox-gl-js/issues/981 and https://github.com/mapbox/mapbox-gl-js/issues/4180 are related, except they're for camera movements

## Design Alternatives

My current workaround is simply to manipulate the source geometry (via [turf.bezierSpline](http://turfjs.org/docs/#bezierSpline) and the like), adding more vertices to improve perceived smoothness.

## Design

The current workflow is not ideal, because it

1) can greatly bloat the size of the line geometry, and
2) is never actually infinitely smooth (as far as the current screen resolution allows)

[Preliminary](https://webglfundamentals.org/webgl/lessons/webgl-3d-geometry-lathe.html) [reading](https://threejs.org/docs/#api/extras/curves/CubicBezierCurve3) suggests that this is possible on the rendering side, possibly more efficiently than by data manipulation.

### Mock-Up

Some kind of `line-bezier` property, with [dimensionless values](http://cubic-bezier.com/#.17,.67,.83,.67) like in CSS, or [resolution and sharpness](http://turfjs.org/docs/#bezierSpline), or something else.

No implementation file, entry point, or test is named. Start by reviewing the related camera-movement issues and the linked WebGL and Three.js curve references, then identify the rendering path and define the proposed curve API and acceptance criteria. Done should be a concrete, testable scope for rendering smooth lines without source-geometry expansion.

Written by the indexing model from the issue text.