[core] Introduce API to collect placed symbols data

The following methods are added to the `Renderer` class:
- `collectPlacedSymbolData()`
  enables or disables collecting of the placed symbols data

- `getPlacedSymbolsData()`
  if collecting of the placed symbols data is enabled, returns the
  reference to the `PlacedSymbolData` vector holding the collected data.

include/mbgl/renderer/renderer.hpp

@@ -21,6 +21,21 @@ namespace gfx {
 class RendererBackend;
 } // namespace gfx
 
+struct PlacedSymbolData {
+    // Contents of the label
+    std::u16string key;
+    // If symbol contains text, text collision box in viewport coordinates
+    optional<mapbox::geometry::box<float>> textCollisionBox;
+    // If symbol contains icon, icon collision box in viewport coordinates
+    optional<mapbox::geometry::box<float>> iconCollisionBox;
+    // Symbol text was placed
+    bool textPlaced;
+    // Symbol icon was placed
+    bool iconPlaced;
+    // Symbol text or icon collision box intersects tile borders
+    bool intersectsTileBorder;
+};
+
 class Renderer {
 public:
     Renderer(gfx::RendererBackend&, float pixelRatio_, const optional<std::string>& localFontFamily = {});
@@ -54,12 +69,32 @@ public:
     void getFeatureState(FeatureState& state, const std::string& sourceID, const optional<std::string>& sourceLayerID,
                          const std::string& featureID) const;
 
-    void removeFeatureState(const std::string& sourceID, const optional<std::string>& sourceLayerID,
-                            const optional<std::string>& featureID, const optional<std::string>& stateKey);
+    void removeFeatureState(const std::string& sourceID,
+                            const optional<std::string>& sourceLayerID,
+                            const optional<std::string>& featureID,
+                            const optional<std::string>& stateKey);
 
     // Debug
     void dumpDebugLogs();
 
+    /**
+     * @brief In Tile map mode, enables or disables collecting of the placed symbols data,
+     * which can be obtained with `getPlacedSymbolsData()`.
+     *
+     * The placed symbols data collecting is disabled by default.
+     */
+    void collectPlacedSymbolData(bool enable);
+
+    /**
+     * @brief If collecting of the placed symbols data is enabled, returns the reference
+     * to the `PlacedSymbolData` vector holding the collected data.
+     *
+     * Note: the returned vector gets re-populated at every `render()` call.
+     *
+     * @return collected placed symbols data
+     */
+    const std::vector<PlacedSymbolData>& getPlacedSymbolsData() const;
+
     // Memory
     void reduceMemoryUse();
     void clearData();