[core] Port "collision group" plumbing to gl-native.

[node] Hook up map-wide "crossSourceCollisions" option, defaulting to true.
[test] Pass "crossSourceCollisions" test option through test harness; enable cross-source-collisions tests on native.

platform/node/src/node_map.hpp

@@ -82,6 +82,7 @@ public:
 
     const float pixelRatio;
     mbgl::MapMode mode;
+    bool crossSourceCollisions;
     NodeThreadPool threadpool;
     NodeMapObserver mapObserver;
     std::unique_ptr<mbgl::HeadlessFrontend> frontend;