add grid module, and change some functions in layer-tree.sml to use it instead of having grid-logic in there

fcore/grid.sml

@@ -0,0 +1,23 @@
+structure Grid =
+struct
+  type square = {r: int, g: int, b: int, a: int}
+
+  type t = square vector vector
+
+  val emptyPixel = {r = 0, g = 0, b = 0, a = 0}
+
+  fun isBlank ({a, ...}: square) = a = 0
+
+  fun changeGridSize maxSide grid =
+    Vector.tabulate (maxSide, fn i =>
+      if i < Vector.length grid then
+        let
+          val yAxis = Vector.sub (grid, i)
+        in
+          Vector.tabulate (maxSide, fn ii =>
+            if ii < Vector.length yAxis then Vector.sub (yAxis, ii)
+            else emptyPixel)
+        end
+      else
+        Vector.tabulate (maxSide, fn _ => emptyPixel))
+end