some turret changes from the XT branch

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@11116 387891d4-d844-0410-90c0-e4c51a9137d3
this too

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@11117 387891d4-d844-0410-90c0-e4c51a9137d3
from xt branch: add some hysteresis to asteroid fields to prevent "popping" when an asteroid exceeds the bounds

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@11118 387891d4-d844-0410-90c0-e4c51a9137d3