add functions to add and remove pixels from layer-tree

fcore/grid.sml

@@ -1,12 +1,12 @@
 structure Grid =
 struct
-  type square = {r: int, g: int, b: int, a: int}
+  type pixel = {r: int, g: int, b: int, a: int}
 
-  type t = square vector vector
+  type t = pixel vector vector
 
   val emptyPixel = {r = 0, g = 0, b = 0, a = 0}
 
-  fun isBlank ({a, ...}: square) = a = 0
+  fun isBlank ({a, ...}: pixel) = a = 0
 
   fun changeGridSize maxSide grid =
     Vector.tabulate (maxSide, fn i =>
@@ -20,4 +20,16 @@ struct
         end
       else
         Vector.tabulate (maxSide, fn _ => emptyPixel))
+
+  fun updateGrid (grid, newX, newY, pixel) =
+    let
+      val yAxis = Vector.sub (grid, newX)
+      val yAxis = Vector.update (yAxis, newY, pixel)
+    in
+      Vector.update (grid, newX, yAxis)
+    end
+
+  fun makeEmpty maxSide =
+    Vector.tabulate (maxSide, fn _ =>
+      Vector.tabulate (maxSide, fn _ => emptyPixel))
 end