fix for mouse sensitivity in full screen mode in linux

[Paul Hayes]

Re-installed this port today (glad to see there is still interest and signs of development!).  I can't remember if it was the same last time I tried it but I found the mouse sensitivity too be far too high in full screen mode.  It was OK in windowed mode.

I am using Debian Jessie and have already applied the same speed max fps patch.

I discovered that it's actually an SDL peculiarity rather than anything with swars itself.  

The fix is to run:

export SDL_MOUSE_RELATIVE=0

Before starting the game.  Add that line to your .bashrc file and then it's always applied.

cheers,

Paul.