Added custom radii for each corner of RRect. (#2820)

In order to implement more detailed animation in Flutter, precise
handling of the rounded corner's radii is necessary. This commit
adds a data structure Radius and updated RRect accordingly.

flutter/lib/ui/painting/rrect.cc

@@ -20,9 +20,14 @@ RRect DartConverter<RRect>::FromDart(Dart_Handle value) {
   if (buffer.data() == nullptr)
     return result;
 
-  result.sk_rrect.setRectXY(
+  SkVector radii[4] = {
+    { buffer[4], buffer[5] }, { buffer[6], buffer[7] },
+    { buffer[8], buffer[9] }, { buffer[10], buffer[11] }
+  };
+
+  result.sk_rrect.setRectRadii(
       SkRect::MakeLTRB(buffer[0], buffer[1], buffer[2], buffer[3]),
-      buffer[4], buffer[5]);
+      radii);
 
   result.is_null = false;
   return result;