Visibility as a property on the Features for a GeoJSON source

[steinbachr]

Given a GeoJSON source with a bunch of features we want to render as symbols, it would be good to have a property appended to each feature, isVIsible, designating whether that feature is currently visible or not. The issue is especially important for cases when trying to select features within a given boundary by using featuresAt or featuresWithin. Without this identifier, it's impossible to tell whether the features selected are visible or currently being hidden without doing the extra and - seemingly unnecessary - steps of:

1. checking which of the selected features intersect each other at the current zoom
2. for those that intersect, comparing drawing order and only selecting the feature drawn first

See fiddle here for example of issue.

Note: This all assumes icon-allow-overlap: false on the layer, otherwise this is a non-issue

[jfirebaugh]

Duplicate of #582.