[core] Fix alpha handling in rgba and to-rgba operators

platform/node/test/ignores.json

@@ -1,8 +1,5 @@
 {
   "expression-tests/coalesce/inference": "https://github.com/mapbox/mapbox-gl-native/issues/10588",
-  "expression-tests/interpolate/linear-color": "https://github.com/mapbox/mapbox-gl-native/issues/10604",
-  "expression-tests/to-rgba/alpha": "https://github.com/mapbox/mapbox-gl-native/issues/10604",
-  "expression-tests/to-rgba/basic": "https://github.com/mapbox/mapbox-gl-native/issues/10604",
   "query-tests/circle-stroke-width/inside": "https://github.com/mapbox/mapbox-gl-native/issues/10307",
   "query-tests/geometry/multilinestring": "needs investigation",
   "query-tests/geometry/multipolygon": "needs investigation",

src/mbgl/style/expression/compound_expression.cpp

@@ -210,7 +210,12 @@ std::unordered_map<std::string, CompoundExpressionRegistry::Definition> initiali
         );
     });
     define("to-rgba", [](const Color& color) -> Result<std::array<double, 4>> {
-        return std::array<double, 4> {{ color.r, color.g, color.b, color.a }};
+        return std::array<double, 4> {{ 
+            255 * color.r / color.a, 
+            255 * color.g / color.a,
+            255 * color.b / color.a, 
+            color.a 
+        }};
     });
     
     define("rgba", rgba);

src/mbgl/style/expression/util.cpp

@@ -1,4 +1,3 @@
-
 #include <mbgl/style/expression/util.hpp>
 #include <mbgl/style/expression/value.hpp>
 
@@ -30,10 +29,9 @@ Result<Color> rgba(double r, double g, double b, double a) {
             "]: 'a' must be between 0 and 1."
         };
     }
-    return Color(r / 255, g / 255, b / 255, a);
+    return Color(r / 255 * a, g / 255 * a, b / 255 * a, a);
 }
 
 } // namespace expression
 } // namespace style
 } // namespace mbgl
-

src/mbgl/util/color.cpp

@@ -24,9 +24,9 @@ optional<Color> Color::parse(const std::string& s) {
 
 std::string Color::stringify() const {
     return "rgba(" +
-        util::toString(r * 255) + "," +
-        util::toString(g * 255) + "," +
-        util::toString(b * 255) + "," +
+        util::toString(r * 255 / a) + "," +
+        util::toString(g * 255 / a) + "," +
+        util::toString(b * 255 / a) + "," +
         util::toString(a) + ")";
 }