add layer field to app type, as preparation for switching to layer tree

fcore/layer-tree.sml

@@ -1,16 +1,19 @@
 structure LayerTree =
 struct
   type square = {r: int, g: int, b: int, a: int}
-  val emptyPixel = {r = 0, g = 0, b = 0, a = 0}
 
   type grid = square vector vector
 
-  fun isBlank ({a, ...}: square) = a = 0
-
   datatype tree =
     NODE of {key: int, value: grid, left: tree, right: tree}
   | LEAF
 
+  val emptyPixel = {r = 0, g = 0, b = 0, a = 0}
+
+  val emptyTree = LEAF
+
+  fun isBlank ({a, ...}: square) = a = 0
+
   fun insert (newKey, newValue, tree) =
     case tree of
       LEAF => NODE {key = newKey, value = newValue, left = LEAF, right = LEAF}