Fix built-in wxPrintf() for repeated positional parameters

Allow wxPrintf("%1$s %1$s", "foo") to work.

Take into account the possibility that the number of format specifiers
and the number of actual arguments can be different.

Closes #9367.

tests/strings/strings.cpp

@@ -188,6 +188,8 @@ void StringTestCase::Format()
         "4 world hello world 3",
         wxString::Format("%4$d %2$s %1$s %2$s %3$d", "hello", "world", 3, 4)
     );
+
+    CHECK( wxString::Format("%1$o %1$d %1$x", 20) == "24 20 14" );
 }
 
 void StringTestCase::FormatUnicode()