Stupid hack: force an entity frame once 5 frames were skipped.

Stupid hack: force an entity frame once 5 frames were skipped.
Fixes issues with cl_movement replay taking longer and longer when standing still and nothing on the map moves, while not causing so many empty entity frames that replay of lost frames can cause problems (well, in theory it now can, if an entity takes 5+3 server frames to be processed on the client, that is 0.1sec at a sys_ticrate matching 72fps - so try to avoid doing overlong processing in CSQC entity receive functions).

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@9104 d7cf8633-e32d-0410-b094-e92efae38249