added polygonoffset as a texture property, used on submodels (such as

added polygonoffset as a texture property, used on submodels (such as
doors in q1bsp maps) to prevent zfighting if they exactly overlap a
wall, consistent with software quake (which intentionally removed the
portions of the door overlapping a wall)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@7375 d7cf8633-e32d-0410-b094-e92efae38249