Fixes and minor enhancements for sub editor

- Zoom now works relative to the mouse's position rather than the center of the screen, much like Valve's Hammer Editor (or pulsegun's map editor shameless plug ;) )
- Rectangles now have line widths dependent on the camera zoom, not perfect but it's better than having lines disappear
- Fixed background resizing
- Fixed editor being completely broken because of no vsync, might still have a few bugs here and there
- Fixed selection rectangle not rendering at all when dragging from bottom right to top left
- Newline change in Item.cs

Subsurface/Source/Screens/Screen.cs

@@ -28,6 +28,11 @@ namespace Barotrauma
             selected = this;
         }
 
+        public virtual Camera Cam
+        {
+            get { return null; }
+        }
+
         public virtual void Update(double deltaTime)
         {
         }