add scaffolding in imperative shell to help update canvas width and height

fcore/normal-mode.sml

@@ -252,8 +252,9 @@ struct
   fun updateBlue model = (AppWith.b model, [])
   fun updateAlpha model = (AppWith.a model, [])
 
-  fun updateCanvasWidth (model, newCanvasWidth) =
+  fun updateCanvasWidth model =
     let
+      val newCanvasWidth = #modalNum model
       val model = AppWith.canvasWidth (model, newCanvasWidth)
 
       val
@@ -322,7 +323,7 @@ struct
     | KEY_G => updateGreen model
     | KEY_B => updateBlue model
     | KEY_A => updateAlpha model
-    | KEY_W newCanvasWidth => updateCanvasWidth (model, newCanvasWidth)
+    | KEY_W => updateCanvasWidth model
     | RESIZE_WINDOW {width, height} => resizeWindow (model, width, height)
     | UNDO_ACTION => undoAction model
     | REDO_ACTION => redoAction model