[core] Fix transition to default style when no classes are applied

Some property transitions were being set to their fallback values when
no classes are being used. This issue is particularly perceived when the
default transition duration is not zero (animated transitions).

src/mbgl/style/style_layer.cpp

@@ -39,6 +39,12 @@ void StyleLayer::setClasses(const std::vector<std::string> &class_names, const T
     // Stores all keys that we have already added transitions for.
     std::set<PropertyKey> already_applied;
 
+    // We only apply the default style values if there are no classes set.
+    if (class_names.empty()) {
+        applyClassProperties(ClassID::Default, already_applied, now, defaultTransition);
+        return;
+    }
+
     // Reverse iterate through all class names and apply them last to first.
     for (auto it = class_names.rbegin(); it != class_names.rend(); ++it) {
         const std::string &class_name = *it;
@@ -62,14 +68,16 @@ void StyleLayer::setClasses(const std::vector<std::string> &class_names, const T
 
         AppliedClassPropertyValues &appliedProperties = property_pair.second;
         // Make sure that we don't do double transitions to the fallback value.
-        if (appliedProperties.mostRecent() != ClassID::Fallback) {
-            // This property key hasn't been set by a previous class, so we need to add a transition
-            // to the fallback value for that key.
-            const TimePoint begin = now + defaultTransition.delay;
-            const TimePoint end = begin + defaultTransition.duration;
-            const PropertyValue &value = PropertyFallbackValue::Get(key);
-            appliedProperties.add(ClassID::Fallback, begin, end, value);
+        if (appliedProperties.mostRecent() == ClassID::Fallback) {
+            continue;
         }
+
+        // This property key hasn't been set by a previous class, so we need to add a transition
+        // to the fallback value for that key.
+        const TimePoint begin = now + defaultTransition.delay;
+        const TimePoint end = begin + defaultTransition.duration;
+        const PropertyValue &value = PropertyFallbackValue::Get(key);
+        appliedProperties.add(ClassID::Fallback, begin, end, value);
     }
 }