[core][node] Serialize formatted expression's text-color option

This enables proper round-tripping for serialized format expression.

include/mbgl/util/color.hpp

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <mbgl/util/optional.hpp>
+#include <mbgl/util/feature.hpp>
 
 #include <cassert>
 #include <string>
@@ -39,6 +40,7 @@ public:
     static optional<Color> parse(const std::string&);
     std::string stringify() const;
     std::array<double, 4> toArray() const;
+    mbgl::Value toObject() const;
 };
 
 inline bool operator==(const Color& colorA, const Color& colorB) {

platform/node/src/node_expression.cpp

@@ -192,6 +192,11 @@ struct ToValue {
             } else {
                 serializedSection.emplace("fontStack", mbgl::NullValue());
             }
+            if (section.textColor) {
+                serializedSection.emplace("textColor", section.textColor->toObject());
+            } else {
+                serializedSection.emplace("textColor", mbgl::NullValue());
+            }
             sections.emplace_back(serializedSection);
         }
         serialized.emplace("sections", sections);

src/mbgl/util/color.cpp

@@ -44,4 +44,11 @@ std::array<double, 4> Color::toArray() const {
     }
 }
 
+mbgl::Value Color::toObject() const {
+    return std::unordered_map<std::string, mbgl::Value>{{"r", double(r)},
+                                                        {"g", double(g)},
+                                                        {"b", double(b)},
+                                                        {"a", double(a)}};
+}
+
 } // namespace mbgl