CARTO: Automatically add labels to line & polygon layers

[felixpalmer]

Background

It is often desirable to add labels to line or polygon layers, but calculating the positioning is not trivial - especially if the data is being loaded tile by tile.

This PR add functionality to the VectorTileLayer to automatically add labels when the autoLabels prop is set.

Usage:

const vectorTileLayer = new VectorTileLayer({
  // Configure as normal
  data: <LINE or POLYGON data>,
  ...,
  // Add props for text rendering
  getText: f => f.properties.name
});

Doc PR to follow

Change List

• Added new utils file label-utils that calculates label positions
• Tests for label-utils
• Tidy types and rework common code

[coveralls]

coverage: 91.574% (-0.03%) from 91.607%
when pulling 9fe06b4 on felix/carto-line-poly-labels
into 31132ef on master.