Instead of indexing all features for querying, index only those that are
actually within the tile boundaries. This prevents duplicate results
from being returned for point features. This is only a partial fix for
the duplicate results problem (lines and polygons can still return
duplicate results).

…e of consistency with vector tiles), necessitating splitting MultiLineString features into individual LineStrings so they can hold their own distance attributes

* Add render test

* Add unit tests for color ramp rendering

…s), clean up style-spec ref, add validation to ensure gradients are only used with geojson sources

…[ci skip]