remove 'squares' field from app type, and refactor most functions in common-update.sml to use layer tree instead of squares vector

fcore/app-init.sml

@@ -23,12 +23,9 @@ struct
           (windowWidth, windowHeight, canvasWidth, canvasHeight)
 
       val maxPoints = Int.max (canvasWidth, canvasHeight)
-      val squares = Vector.tabulate (maxPoints, fn _ =>
-        Vector.tabulate (maxPoints, fn _ => {r = 0, g = 0, b = 0, a = 0}))
       val layerTree = LayerTree.init maxPoints
     in
       { mode = AppType.NORMAL_MODE
-      , squares = squares
       , canvasWidth = canvasWidth
       , canvasHeight = canvasHeight
       , windowWidth = windowWidth