Added, Visual mode: added support for ROLLCENTER sprite rendering flag,...

Added, Visual mode: added support for ROLLCENTER sprite rendering flag, updated ROLLSPRITE implementation.
Fixed, DECORATE support: the editor was unable to determine actor sprite when the actor itself had no sprites defined and the actor it inherited from was only defined in the game configuration.
Fixed, Drag geometry modes: linedefs without both sides were removed after dragging them when "Replace with Dragged Geometry" mode was active.
Updated ZDoom_DECORATE.cfg.