-Don't try to run too many logic frames if the CPU is already behind. Cap it at a maximum of 4.
-Don't call I_Sleep if there is less than 1ms before the start of the next frame.

git-svn-id: http://odamex.net/svn/root/trunk@4016 b0bcef46-2027-0410-bd56-ab726e9a1d04