This doesn't look stretched to me. http://i.imgur.com/WUB2oJs.png
Topic on User talk:Joylock
I played the first level in the default 800x600 (4:3) and then set the resolution to 2560x1440 (16:9). The 16:9 image looked stretched. I guess maybe it varies based on config?
Did you try 1080p?