Offsetmapping: new "Bias" parameter that sets a custom "null point" instead of always...

Offsetmapping: new "Bias" parameter that sets a custom "null point" instead of always having fixed one (at white), set as 0-255 byte additional "dpoffsetmapping" parm (dpoffsetmapping <technic> <scale> <bias>) a value of 128 makes offsetmapping centered at gray (no effect at gray, black is positive offset, white is negative). Additinally "dpoffsetmapping disabled" now parsed correctly. To see difference of offsetmapping bias see this shot (animated): http://omnicide.legacy-of-kain.ru/work/omnicide000137.gif

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@11309 d7cf8633-e32d-0410-b094-e92efae38249