rsvg2: add method to read huge file (9.5mb+)

I add a method to read huge file like librsvg's rsvg-convert command (--unlimited option).
> Did you mean `flags == RSVG_HANDLE_FLAGS_NONE`?

`RSVG_HANDLE_FLAGS_NONE` is used only when initialize.

> But why do you use `if`? We can always use `rsvg_handle_new_from_gfile_sync()`.

I have a conflict in this line.
I think using  `rsvg_handle_new_from_gfile_sync()` is better but I think `rsvg_handle_new_from_gfile_sync()` has bug because this function always pass huge file (maybe librsvg's bug).
It's is mainly reason "I use `if`" but I don't care about using `rsvg_handle_new_from_gfile_sync()`.