[prey] SDL issue

[Ryan C. Gordon]

> SDL button 8 exceeds supported range
>
> Did anyone come across this?

This appears to be a bug: idTech 4 (or maybe just Prey?) should support 
8 mouse buttons, not counting the two that X11 uses for the mouse wheel, 
but the current code has this incorrect test.

I'll look into it for the next build. In the meantime, you could try 
remapping those mouse buttons to keys or lower button values to fool Prey.

--ryan.