Use top-level layer name as default repeat_group name

[nvkelso]

Sister issue to Tangram JS https://github.com/tangrams/tangram/pull/687.

• Prior behavior for the repeat_group parameter has been to default to the full, unique combination of all layer names that a given feature matches. While designed to allow for features with identical names but different purposes (e.g. a road and nearby park with the same name) to both properly display without repeat interference between them, in practice this behavior is too granular, causing many crowded, repeated labels for more common cases such as a road that changes from major to minor classification, while retaining its name. This sub-optimal behavior has been hidden by a bug that did not properly implement the original intent.
• In re-evaluating the desired behavior, the default has been changed so that a feature's default repeat_group is that of its top-level parent in the scene's layers. For example, for typical scene structures this means that all roads will belong to the same repeat group, as would all landuse, all POIs, etc. (assuming top-level layers of roads, landuse, pois, and so forth). This better captures the original intent. In practice (combined with the prior long-standing bug), most scenes will not be affected by this change, though it may have a subtle impact on label placement and density.