Description: Save an area of the graphics screen
Mapped Command: SAVE GRAPHICS SCREEN
FUNCTION __SaveGScreen(label,Pos1a,Pos1b,Pos2a,Pos2b)
   local handle
   local screen := __UnusedScreen(label)
   if screen > 0
      handle = snapcopy(__XdGE(Pos1b),__YdGE(Pos2a),__XdGE(Pos2b),__YdGE(Pos1a),0)
      if handle != 0
         _screens_[screen,1] := Pos1a            // upper left row
         _screens_[screen,2] := Pos1b            // upper left column
         _screens_[screen,3] := Pos2a            // lower right row
         _screens_[screen,4] := Pos2b            // lower right column
         _screens_[screen,5] := handle           // dGE video handle
         _screens_[screen,6] := label            // screen label
      else
         __HandleError(NoMemoryLeft,label)
      endif
   else
      __HandleError(NoHandlesLeft,label)
   endif
RETURN(screen)