Add optional pixel radius for feature selection

[bcamper]

This PR adds an optional pixel radius when requesting features at/near a pixel location. Previously, feature selection only returned a feature at the exact pixel location provided, but there are common use cases where a larger radius is useful, such as:

• Mobile devices where input is not always so precise
• Features such as thin lines may be difficult to select

The following methods are affected:

• Direct scene interface:
  • scene.getFeatureAt(pixel, { radius })
• Leaflet layer interface for setting selection handlers
  • layer.setSelectionEvents (events, { radius })
• Leaflet layer instantiation:

  layer = Tangram.leafletLayer({
    scene: 'path/to/scene.yaml',
    events: {
       click: clickHandler,
    },
    selectionRadius: 10
  };

In all cases, radius or selectionRadius is an optional value in pixels (default matches existing behavior, where radius is zero). When using a radius, the feature closest to the center point will be returned. (As with existing feature selection, only features marked as interactive: true will register.)

[bcamper]

Released in v0.13.0