Remove 1280x1000 scaling.

We already have 1280x960, which is the correct aspect ratio. This
means that when running at 1080p the windowboxing borders will be
slightly thicker, but if we're already showing borders anyway, it's
better to at least use the correct aspect ratio.

This fixes #460. Thanks to Doom_user for asking about this on
Doomworld: http://www.doomworld.com/vb/post/1316735