require mapCopyright to be set

lib/Properties.hs

@@ -68,6 +68,9 @@ checkMap = do
   unless (tiledmapTileheight tiledmap == 32 && tiledmapTilewidth tiledmap == 32)
     $ complain "The map's tile size must be 32 by 32 pixels."
 
+  unlessHasProperty "mapCopyright"
+    $ error "must give the map's copyright via the \"mapCopyright\" property."
+
 -- | Checks a single property of a map.
 --
 -- Doesn't really do all that much, but could in theory be expanded into a