Constrain the window's aspect ratio

I was seeing the RecastDemo window span the full width of both of my monitors under Linux.

SDL 1.2 does not appear to provide detailed information about multi-monitor configuration, so I've applied the following changes to get a better starting size:
- Prevent the window from having an aspect ratio wider than 16:9
- Reduce both the width and the height by 80 pixels (I keep my task bar on the right)

This makes the RecastDemo window size more useful for me. I can't see that it would be significantly worse for other monitor configurations.