roundedbox: Only grow border-radius if there is one

This is relevant for shadow spread and goes in line with the web.

Testcase included.

testsuite/reftests/box-shadow-spread.css

@@ -0,0 +1,9 @@
+@import "reset-to-defaults.css";
+
+.test {
+  box-shadow: 0 0 0 10px red;
+}
+
+.reference {
+  border: solid 10px red;
+}