[core] Fixed to-rgba with zero opacity

include/mbgl/util/color.hpp

@@ -4,6 +4,7 @@
 
 #include <cassert>
 #include <string>
+#include <array>
 
 namespace mbgl {
 
@@ -37,6 +38,7 @@ public:
 
     static optional<Color> parse(const std::string&);
     std::string stringify() const;
+    std::array<double, 4> toArray() const;
 };
 
 inline bool operator==(const Color& colorA, const Color& colorB) {

src/mbgl/style/expression/compound_expression.cpp

@@ -209,12 +209,7 @@ std::unordered_map<std::string, CompoundExpressionRegistry::Definition> initiali
         );
     });
     define("to-rgba", [](const Color& color) -> Result<std::array<double, 4>> {
-        return std::array<double, 4> {{ 
-            255 * color.r / color.a, 
-            255 * color.g / color.a,
-            255 * color.b / color.a, 
-            color.a 
-        }};
+        return color.toArray();
     });
     
     define("rgba", rgba);

src/mbgl/style/expression/value.cpp

@@ -106,12 +106,13 @@ Value ValueConverter<mbgl::Value>::toExpressionValue(const mbgl::Value& value) {
 mbgl::Value ValueConverter<mbgl::Value>::fromExpressionValue(const Value& value) {
     return value.match(
         [&](const Color& color)->mbgl::Value {
+            std::array<double, 4> array = color.toArray();
             return std::vector<mbgl::Value>{
                 std::string("rgba"),
-                double(255 * color.r / color.a),
-                double(255 * color.g / color.a),
-                double(255 * color.b / color.a),
-                double(color.a)
+                array[0],
+                array[1],
+                array[2],
+                array[3],
             };
         },
         [&](const std::vector<Value>& values)->mbgl::Value {

src/mbgl/util/color.cpp

@@ -23,11 +23,25 @@ optional<Color> Color::parse(const std::string& s) {
 }
 
 std::string Color::stringify() const {
+    std::array<double, 4> array = toArray();
     return "rgba(" +
-        util::toString(r * 255 / a) + "," +
-        util::toString(g * 255 / a) + "," +
-        util::toString(b * 255 / a) + "," +
-        util::toString(a) + ")";
+        util::toString(array[0]) + "," +
+        util::toString(array[1]) + "," +
+        util::toString(array[2]) + "," +
+        util::toString(array[3]) + ")";
+}
+
+std::array<double, 4> Color::toArray() const {
+    if (a == 0) {
+        return {{ 0, 0, 0, 0 }};
+    } else {
+        return {{
+            r * 255 / a,
+            g * 255 / a,
+            b * 255 / a,
+            a,
+        }};
+    }
 }
 
 } // namespace mbgl