Skip some redundant updates to the grid and spotlight

While debugging our layout shift issue I learned that a single change to the sort order of the participants can cause 3 or 4 redundant emissions of the same items in the same order. Since each of these would cause React to re-render the grid, skipping these spurious emissions seems like an easy performance win.

src/utils/array.ts

@@ -0,0 +1,16 @@
+/*
+Copyright 2024 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only
+Please see LICENSE in the repository root for full details.
+*/
+
+/**
+ * Determine whether two arrays are equal by shallow comparison.
+ */
+export function shallowEquals<A>(first: A[], second: A[]): boolean {
+  if (first.length !== second.length) return false;
+  for (let i = 0; i < first.length; i++)
+    if (first[i] !== second[i]) return false;
+  return true;
+}