Exclusive and Priority for SceneLayer

[tallytalwar]

• Sort SceneLayers such that all exclusive layers come before
  non-exclusive layers and in order of increasing priority

TODO:

• Add unit test covering priority and exclusive properties.

Resolves #2036

[matteblair]

After a year in limbo, this PR is finally ready!

This now implements the exclusive and priority parameters for scene layers as described in #2036. I also added tests for this feature and refactored a lot of the surrounding code to simplify logic and improve clarity.

The scene layer sorting in this implementation looks a bit different than that in the Tangram JS implementation (tangrams/tangram#705). This is because we have a different way of traversing the layer tree when evaluating rules for a feature. Basically, when draw rules are being merged we keep the first value that is set for a parameter, unless the new one is from a greater depth. So, for sibling layers to follow the rules of exclusive and priority, we just sort them by exclusivity first, then by priority, and finally by fully-qualified name (in reverse-alphabetical order).

[matteblair]

This is looking good to go!

The one thing I've found from starting to use this feature is that I frequently encounter layers where every sublayer can or should be exclusive: true. This is common when a set of sublayers categorize features by a single tag like kind. It would be convenient in these cases to have a shorthand like sublayers_exclusive: true to write once on the parent layer in these cases.

[tallytalwar]

Sorry COVID-19 situation means I rarely get cycles to check tangram notifications. This looks good to me and thanks a lot @matteblair for continuing this work from where I left off!