fix loading to use and return layer tree

message-types/input-msg.sml

@@ -31,11 +31,7 @@ struct
   | ARROW_DOWN
   | KEY_ENTER
   | KEY_SPACE
-  | USE_SQUARES of
-      { squares: {r: int, g: int, b: int, a: int} vector vector
-      , canvasWidth: int
-      , canvasHeight: int
-      }
+  | USE_LAYERS of {tree: LayerTree.t, canvasWidth: int, canvasHeight: int}
   | SQUARES_LOAD_ERROR
   | FILE_BROWSER_AND_PATH of
       {fileBrowser: AppType.file_browser_item vector, path: string}