Reduce the MTU size to deal with Windows networking issue.

In the course of testing, I found that when downloading metavoxel terrain from a server to a Windows client over the LAN, about every other packet was dropped when the MTU was at 1500.  Lowering it to 1450 magically resolved the issue.  Hopefully, this will also help with bandwidth usage on EC2 servers, and if it does, we may want to consider lowering the MTU everywhere (not just in the metavoxel code).