Force modestmaps preview to 256x256 tile dimensions

When using tiles with a `tile height` of something other than 256px, the builtin preview got all wonky.  The tiles were being rendered at their native size but modestmaps was arranging them in a typical 256px layout.  So if you have 512px retina tiles, they would overlap.

This is a pretty dirty hack to constrain the images to the 256x256 layout that modestmaps uses (and display retina tiles properly).