[tile mode] Improvements in symbol placement on tile borders

This change fixes the following problems:
1) Before this change, when multiple symbols were grouped close to the
tile borders, different tiles could place different symbols, because each
tile arbitrary assigned feature ids, and these ids defined the placement
order being applied. This caused artifacts at the tile boundaries.

With this change, in tile mode the placement order of two symbols crossing
a tile border is defined by their anchor `y` values. It means that the
symbols are being placed the same order across all the tiles.

2) Before this change, the engine did not place with priority the symbols
crossing the borders between two neighboring tiles. Now it does, and it
improves a lot symbol placement stability in the tile map mode.

1 files changed, 5 insertions, 6 deletions

diff --git a/src/mbgl/text/collision_index.hpp b/src/mbgl/text/collision_index.hpp
index b9f3e9d88a..2ed3ab81bc 100644
--- a/src/mbgl/text/collision_index.hpp
+++ b/src/mbgl/text/collision_index.hpp
@@ -21,12 +21,11 @@ public:
     using CollisionGrid = GridIndex<IndexedSubfeature>;
 
     explicit CollisionIndex(const TransformState&, MapMode);
-    bool featureIntersectsTileBorders(const CollisionFeature& feature,
-                                      Point<float> shift,
-                                      const mat4& posMatrix,
-                                      const float textPixelRatio,
-                                      const CollisionBoundaries& tileEdges) const;
-
+    bool intercectsTileEdges(const CollisionBox&,
+                             Point<float> shift,
+                             const mat4& posMatrix,
+                             const float textPixelRatio,
+                             const CollisionBoundaries& tileEdges) const;
     std::pair<bool, bool> placeFeature(
         const CollisionFeature& feature,
         Point<float> shift,