- Fixed more possible NULL derefs from r4264

git-svn-id: http://mancubus.net/svn/hosted/zdoom/zdoom/trunk@4290 a95907f1-e10c-0410-b46f-9e587ccb1026
- Move bitdepth check outside the loop for grayscale unpacking. Also, don't use a multiply for
  1-bit pixels. Use a lookup table for 2-bit pixels, because it's probably faster than a bunch
  of shifts and register juggling.

git-svn-id: http://mancubus.net/svn/hosted/zdoom/zdoom/trunk@4291 a95907f1-e10c-0410-b46f-9e587ccb1026
- Added ACS functions SetCVarString and SetUserCVarString. These work just like their non-string
  counterparts except that their value argument is an ACS string. (Note that they work with any
  type of cvar, not just string cvars.)
- Make UCVarValue::String point to a constant string.

git-svn-id: http://mancubus.net/svn/hosted/zdoom/zdoom/trunk@4292 a95907f1-e10c-0410-b46f-9e587ccb1026
- Added ACS functions GetCVarString and GetUserCVarString. These act like their non-string
  counterparts, except that they return strings. Like strparam, the strings they return are
  only guaranteed to be valid for the tick they are called during. (Note that these work with any
  cvar, not just string ones.)

git-svn-id: http://mancubus.net/svn/hosted/zdoom/zdoom/trunk@4293 a95907f1-e10c-0410-b46f-9e587ccb1026