Consider 1px overlap by default instead of 0

Comment from @gasi :

> Love that you are building this tool to help the zooming community!
> 
> Psst, please consider using Overlap="1" as default. See this blog post for details https://web.archive.org/web/20180128073409/http://benvanik.tumblr.com/post/13978156533/tile-overlap
>
> Let me know if you have any questions