add interpolation

include/llmr/style/applied_class_properties.hpp

@@ -34,6 +34,10 @@ public:
     void add(ClassID class_id, timestamp begin, timestamp end, const PropertyValue &value) {
         properties.emplace_back(class_id, begin, end, value);
     }
+
+    bool hasTransitions() const {
+        return properties.size() > 1;
+    }
 };
 
 }

include/llmr/style/style.hpp

@@ -43,6 +43,8 @@ public:
     // of applied classes.
     void updateClasses();
 
+    bool hasTransitions() const;
+
 public:
     std::shared_ptr<Sprite> sprite;
     std::shared_ptr<StyleLayerGroup> layers;

include/llmr/style/style_layer.hpp

@@ -42,6 +42,8 @@ public:
     void setClasses(const std::vector<std::string> &class_names, timestamp now,
                     const PropertyTransition &defaultTransition);
 
+    bool hasTransitions() const;
+
 private:
     // Applies all properties from a class, if they haven't been applied already.
     void applyClassProperties(ClassID class_id, std::set<PropertyKey> &already_applied,

include/llmr/style/style_layer_group.hpp

@@ -13,6 +13,7 @@ public:
                     const PropertyTransition &defaultTransition);
     void updateProperties(float z, timestamp t);
 
+    bool hasTransitions() const;
 public:
     std::vector<std::shared_ptr<StyleLayer>> layers;
 };