Add fallback placement for falled link based placement spots

src/Dodge/LevelGen/Data.hs

@@ -32,6 +32,7 @@ data PlacementSpot
     = PS { _psPos :: Point2 , _psRot :: Float }
     | PSLnk { _psLinkTest :: (Point2,Float) -> Bool
             , _psLinkShift :: (Point2,Float) -> (Point2,Float)
+            , _psFallback :: Maybe Placement
             }
     | PSRoomRand
         { _psRoomRandPointNum :: Int }