startLayer can actually be set to `false`

(in which case it just doesn't do anything at all)

lib/Properties.hs

@@ -343,10 +343,9 @@ checkTileLayerProperty p@(Property name _value) = case name of
     "startLayer" -> do
       forbidEmptyLayer
       layer <- askContext
-      offersEntrypoint $ layerName layer
       unwrapBool p $ \case
-        True  -> pure ()
-        False -> complain "property \"startLayer\" must be set to true."
+        True  -> offersEntrypoint $ layerName layer
+        False -> warn "property \"startLayer\" is useless if set to false."
     "silent" -> do
       isBool p
       uselessEmptyLayer