Attribution for data sources

[jjhembd]

Delegated to tile-setter from GlobeletJS/GlobeletJS#16.

tile-setter should automatically add a small overlay, with info from the .attribution property of the TileJSON, for each source in the style document that is currently being displayed.

[jjhembd]

sources.js currently checks the visible status of each layer. However, this property is not updated based on whether we are between the min/maxzoom for that layer.

  function loadTilesets() {
    const viewport = coords.getViewport();
    const transform = coords.getTransform();
    grids.forEach(grid => {
      if (!grid.layers.some(l => l.visible)) return;
      tilesets[grid.key] = grid.getTiles(viewport, transform);
    });

We need to add this check to compute an overall status for the current visibility of the source.
Then we can use the source visibility to decide whether to include the attribution for that source.